tachinid flies are killing my Monarch caterpillars...

What can I do???

# jump to comments / feedback / questions below

see also:
raising Monarch caterpillars in Santa Barbara
image gallery of milkweed, Monarch caterpillars, butterflies, etc
Monarch caterpillars eat butternut squash
Monarch caterpillar failed to complete pupating into chrysalis... why?

2017 update - a possible Tachinid fly solution?
Several readers in the comments below have recommended Rescue disposable fly traps (available on Amazon). If you get them and try them out, please let me know how they work!

2020 update - indoor Monarch caterpillar / butterfly enclosures
If you are interested in raising your Monarch caterpillars inside, both to protect them and see them turn into chrysalises and then butterflies, several readers have recommended checking Amazon for their enclosures for raising Monarch caterpillars to butterflies. It looks like Amazon has a good selection of inexpensive, portable, and easily stored caterpillar/butterfly enclosures. Something worth considering...

Floral tubes - Amazon also has this 10 pack of floral tubes for preventing your milkweed cuttings from wilting. Floral tubes are the plastic water-filled tubes that you've probably seen on the end of long-stem roses.

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About the Tachinid fly:
The Tachinid fly (also know as Tachinidae and/or tachina flies or simply tachinids) is a large family of parasitoid flies that lay their eggs in host insects (usually caterpillars). The fly eggs hatch and feed on the host insects, eventually killing the host and emerging as a larva (maggot) that then turns into a fly. One type of Tachinid fly targets the Monarch caterpillar (see images above and below). This Tachinid fly injects its egg or eggs into the Monarch caterpillar, where the eggs hatch and grow as parasoids, eventually killing the caterpillar (or chrysalis). I've actually seen a Tachinid fly land on and inject its egg into a Monarch caterpillar. From what I've witnessed, the fly doesn't really land on and sit on the caterpillar. Instead it's more of a hit-and-run. The fly just touches the caterpillar and leaves, and the caterpillar spasms and drops to the ground, where it writhes and thrashes about, appearing to attempt to rub its back on the ground to dislodge whatever has just hurt it. I've only seen a Tachinid fly hit a caterpillar twice, and it's a blink-of-the-eye thing... usually I just see flies in the area, and then I will notice a caterpillar drop to the ground and wiggle and flop. All of the sources I've read have said the Tachinid fly targets the Monarch caterpillar, but one source said the fly can also inject its egg into a Monarch egg. I am trying to determine if this is true. As you'll also read below, I tried raising two "wild" Monarch caterpillars that were only about a 1/4" long, and they both eventually died from Tachinid maggots. I used to think Tachinid flies only targeted big caterpillars, but from my experience with the 1/4" long caterpillars eventually dying, it is clear to me that Tachinid flies can also target (inject) small Monarch caterpillars. What is unclear is if the Tachinid fly also targets the Monarch egg, as well as what can be done about the dreaded Tachinid fly.

My experience: We've been growing milkweed for Monarch caterpillars in our backyard for about two years. Initially the biggest challenge was just having enough milkweed, which remains a problem still. But lately our biggest problem by far has been the dreaded tachinid fly. We used to lose some caterpillars to the tachinid fly, but lately all of our caterpillars are ending up dead, killed by the tachinid fly. Unfortunately, at this point, I don't have a solution to the problem, but instead am documenting where I stand right now. After finding so many dead caterpillars and no chrysalises, I decided to catch a few caterpillars and bring them inside to safety (and feed them milkweed leaves everyday), so that at least a few caterpillars could become butterflies. I caught three small young caterpillars that were just over a half inch long. But I was surprised and saddened when they got to full size, they died because of tachinid flies. They must have gotten bitten and had fly eggs injected into them before I caught them. I also accidentally found two very small caterpillars on the back of milkweed leaves that I picked to feed to the bigger caterpillars. Because these two caterpillars were so small (only a 1/4" long) I though for sure they would be ok (not bitten yet), but they both ended up dying after they hung in the "J" shape. So all five caterpillars I brought into the house to "raise" had already been bitten (had eggs injected into them) by tachinid flies. I have since read that tachinid flies may inject their eggs into not only caterpillars but also the Monarch eggs. If this is the case and the flies are injecting all the Monarch eggs with fly eggs, then trying to raise the little caterpillars inside won't help. I also don't really want to raise caterpillars inside anyway, and would much rather just let them feed on milkweed in the "wild" (backyard) and watch nature take it's course, as long as some of the caterpillars can make it and turn into chrysalises and then butterflies. In any event, after having all five caterpillars end up killed by tachinid flies, I found a Monarch egg on a milkweed leaf and brought it in. I'm not sure if I found it before a fly did, but I am hopeful. The egg has since hatched and the now three-day old caterpillar that is about a 1/4" long. I've got my fingers crossed that he will make it, and will update this post as he grows...

UPDATE (October 8, 2015): Success! Well, sort of... the first egg (mentioned above) hatched into a caterpillar that we raised "in captivity" in a screened container, safe from Tachinid flies. The caterpillar grew to full size and successfully turned into a chrysalis, and then hatched into a beautiful Monarch butterfly two days ago. I collected two more eggs immediately after seeing a butterfly lay the eggs, and those two eggs hatched and are now caterpillars that are about an inch long. I've got my fingers crossed on them. If these three eggs all turn into butterflies when the caterpillars are raised "inside" (actually screen containers on our back porch), it would indicate that at least the Tachinid fly is not able to leave it's own eggs on milkweed leaves and then have those eggs hatch into little larva that latch onto passing caterpillars and infect them (since I've been feeding these indoor caterpillars leaves from outdoor milkweed plants). I would assume this is true, but because I've read of other types of tachinid flies that "infect" leaves with their eggs, I want to confirm it's not the case with the Tachinid fly that targets the Monarch caterpillar. [edit - the other two eggs hatched into caterpillars and grew up and never had any Tachinid fly problems, but something unrelated went went wrong and they failed to pupate properly.]

Further, when I was picking a leaf for the two caterpillars we are currently raising (above), I noticed a "just hatched" caterpillar underneath the leaf. This egg was out in the "wild" and was exposed to Tachinid flies. I'm going to raise this caterpillar to see if he makes it. If he makes it, it would indicate Tachinid flies do not target eggs, but if he dies from Tachinid maggots, I will know that a Tachinid fly must have injected it's own egg into this Monarch egg. Stay tuned for an update on this little guy, plus an update on how the last two of the three caterpillars do. As for our wild caterpillars in the backyard, we continue to see many caterpillars of various size, but we haven't found any chrysalises in a long, long time, and it appears that ALL of our "wild" caterpillars are being killed by Tachinid flies.

The image above (taken October 4, 2015) shows a just-hatched Monarch caterpillar I found on an outdoor milkweed leaf that I was picking to feed to an indoor caterpillar we are raising. This egg would have been exposed to Tachinid flies the entire time it was an egg. I am going to raise this caterpillar inside to see it he makes it. If he dies and Tachinid maggots emerge, it would show that Tachinid flies can lay their eggs in Monarch eggs, if not, it could support the idea that Tachinids only target the Monarch caterpillar, and not the Monarch egg. I realize this egg could have been overlooked by the tachinid flies, so if the caterpillar does survive, that doesn't proof Tachinid flies don't target Monarch eggs, but if this caterpillar does die from Tachinids, it would show that it was already "infected" at the time this picture was taken.

update (October 8, 2015): per the picture above, the monarch caterpillar I found outside just after it hatched on October 4, and that I then brought inside, is now 7 days old and seems fine. Did a Tachinid fly inject the Monarch egg while it was outside and before the caterpillar hatched, or will this caterpillar now being raised inside be ok? Time will tell. Stay tuned.

update (October 17, 2015): per the picture above, the monarch caterpillar I found outside after it had just hatched on October 4, and that I then brought inside, is now 13 days old and seems fine, and is probably happy and definitely is fat. Did a Tachinid fly inject the Monarch egg while it was outside and before the caterpillar hatched, or will this caterpillar now being raised inside be ok? Time will tell, but things are looking very good at this point and my assumption now is that the monarch egg was never injected by a tachinid fly. Stay tuned.

update (October 20, 2015): per the picture above, the monarch caterpillar I found outside after it had just hatched on October 4, and that I then brought inside, made its silk button and then hung in a "J" on October 18, and then successfully turned into a beautiful chrysalis on October 19. Stay tuned.

update (October 31, 2015): per the picture above, the monarch caterpillar I found outside after it had just hatched on October 4, and that I then brought inside, successfully emerged from its chrysalis on October 31 as a beautiful Monarch butterfly. At least in this one case, the tachinid flies left the Monarch egg alone and did not inject any of their fly eggs into the Monarch egg. This would indicate that Tachinid flies do not prey on the Monarch eggs, and only prey on the Monarch caterpillars, but again, this is only a study of one egg. In any event, it always gives me great joy to see a new Monarch caterpillar.

update (November 11, 2015... Happy Veterans Day!): per my study above, I had success raising a caterpillar that had just hatched from an egg. But two small (about 1/4" long) caterpillars I tried raising ended up dying from tachinid maggots, indicating they were infected when they were small. So when I saw a very small caterpillar outside, I decided to try raising it inside to see if it would be ok, or if it had already been infected. This caterpillar has been a slow eater. He seems to spend more time hanging out than eating, unlike the other caterpillars. It makes me wonder if he is healthy, but he continues to grow. Here are two pictures of this caterpillar so far, both how big he was when I found him and "rescued" him from the wild and being susceptible to tachinid fly infection, and how he looks at present (day 19). Whereas the caterpillar above pupated on day 15 and turned into a chrysalis, for this fellow it has been 19 days, and he has yet to pupate. He's getting big though, and it looks like he may pupate soon, unless the tachinids get him. Stay tuned...

update (November 17, 2015): per the additional pictures above (day 24 and day 25), the small caterpillar I found outside and brought "inside" to protect him (or her?) from Tachinid flies grew up slowly but sure, but eventually reached full size and on November 16 successfully pupated into a chrysalis! I'm assuming we'll get a butterfly, but I'll update this post if anything goes wrong.

update (December 13, 2015): After the caterpillar in this "study" successfully turned into a chrysalis on November 13, we went away for the week of Thanksgiving, so I clipped the net with the chrysalis attached in a tree in our backyard. I expected the butterfly to emerge while we were gone over Thanksgiving, but when we returned, the chrysalis was still light green. It's been pretty cold since then, especially at night. We were checking the chrysalis on a regular basis, but nothing was happening. Finally, a few days ago the chrysalis started to darken, and yesterday became semi-transparent so we could see the wings. Today, the butterfly finally emerged! It was in the chrysalis for 29 days! This is an indication of how cold weather can really slow down the Monarch butterflies typical lifecycle. In any event, the small caterpillar in this study did manage to avoid the tachinid flies when it was very young, and was then "raised" in "captivity" and is now a healthy, happy Monarch butterfly.

TACHINID FLY SOLUTIONS (as of December 13, 2015):

At this point I'm afraid I only see two solutions if you're experiencing a lot of your outdoor Monarch caterpillars being killed by Tachinid flies. Either (1) raise the caterpillars (from eggs) "inside" or (2) grow a LOT of milkweed and hope for a LOT of caterpillars and hope that a FEW might survive and accept that the vast majority of caterpillars will die.

Solution #1) RAISING CATERPILLARS "INSIDE": The obvious but inconvenient solution is to raise your caterpillars inside and/or in a screened protective environment where Tachinid flies can not get to them. I'm not a fan of this solution because I'd rather see the caterpillars out in nature (our backyard), and I don't like picking milkweed leaves for them every day. The most important thing to remember if you're going to take this approach is that you have to get the Monarch when it is an egg or a just hatched or very, very small caterpillar. If you wait until you find a small caterpillar (about a 1/4" long or bigger) it will probably already be "infected" and it will just eat milkweed, grow a bit, get your hopes up (and your kids' hopes up) and then die! I collected three 1/2" long caterpillars and two 1/4" long caterpillars (five caterpillars total) and raised them in protected containers, but all five died from Tachinid fly maggots! That's where the Tachinid fly photos in the gallery below come from. So the safest approach is to collect Monarch eggs and then have them hatch and grow in a screened protected enclosure.

Solution #2) ACCEPT NATURE. GROW A LOT OF MILKWEED. HOPE FOR A FEW CATERPILLAR SURVIVORS. I'm guessing that one or two caterpillars might have made it in our backyard this year and became a Monarch butterfly, but all we have seen and found outside is dead caterpillars that were killed by Tachinid flies. I've squished countless dead shrunken caterpillars to get the Tachinid fly maggots out (gross!) and then squished and crushed the maggots. From what I have read, in the wild, less than 5% of Monarch eggs will ever survive to become Monarch butterflies. That means 95% won't make it. But in a way, that's just nature maintaining a balance. Because a single Monarch butterfly can lay hundreds of eggs, one can't have a very high survival rate or the Monarch population would overpopulate and wipe out it's food supply, and starve to death. Trying to explain this to a kid can be hard though. I have read that Tachinid flies normally kill about 20% of Monarch caterpillars before they become butterflies, but Monarchs face a host of other predators and diseases which is why only 5% survive. So if it wasn't a Tachinid fly, it would probably be something else. If you can grow enough milkweed (which can be surprisingly hard considering it's a weed!) and you get a whole lot of caterpillars, odds are at least one or two will make it. In the end, that is nature.

Solution #3) ??? If anyone knows of any way of fighting off Tachinid flies other that the two suggestions above, please let me know with a comment below. One should also keep in mind that seldom is nature black and white, and that although the Tachinid fly kills Monarch caterpillars, it also kills a lot of pest insects which is why many farmers and gardeners consider the Tachinid fly to be a "beneficial pest".

2017 update - a possible Tachinid fly solution?
Several readers in the comments below have recommended Rescue disposable fly traps (available on Amazon). If you get them and try them out, please let me know how they work!

2019 update - indoor Monarch caterpillar / butterfly enclosures
If you are interested in raising your Monarch caterpillars inside, both to protect them and see them turn into chrysalises and then butterflies, several readers have recommended checking Amazon for their enclosures for raising Monarch caterpillars to butterflies. It looks like Amazon has a good selection of inexpensive, portable, and easily stored caterpillar/butterfly enclosures. Something worth considering...

Try Amazon Prime 30-Day Free Trial
Get FREE 2-day shipping and other prime benefits.
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SPRING 2016 UPDATE - There is hope (April, 2016) ...
Last year (2015) was a disaster. As far as I know, all of our "outdoor" Monarch caterpillars were killed by Tachinid flies. I attempted to bring a few caterpillars inside and raise them in safety, but except for one very tiny caterpillar, all the caterpillars (small and large) I found and attempted to raise inside ended up killed by Tachinids. The only success I had last year was by bringing a few freshly-laid Monarch eggs inside, and raising the resulting indoors-hatched caterpillars in safety.

This year (2016) has started differently. My main tropical milkweed plant got stripped by caterpillars last fall, and was barren over the winter. This spring, it grew a new set of leaves, and then the first "crop" of Monarch caterpillars showed up. I saw no evidence of any Tachinid flies around. They must have died off or left over the winter. In any event, we were blessed with a good dozen big fat happy Monarch caterpillars. A few more small stragglers showed up later after all the leaves were gone, and I had to "relocate" them to our nearby elementary school, which still had some milkweed available. I let the big caterpillars disappear into the yard. We later found two chrysalises in the bushes, and just found a third chrysalis "hiding in plain sight" on our back block brick wall. See the picture of the three chrysalises below. [update continues below pictures]

[spring 2016 update continued]  All three chrysalises hatched into healthy Monarch butterflies. Two of the Monarch butterflies are pictured above, one by his empty chrysalis shell. Our first 2016 "crop"  of Monarchs was a success, with no Tachinid flies around to cause any problems.

So the good news for the start of 2016 is that our Tachinid fly population which plagued us all last year has disappeared over the winter, and our first 2016 "crop" of caterpillars was a success. I'm guessing the Tachinid fly will be back soon and will cause problems for the rest of the year, but we are happy for now. What does this mean for Tachinid fly solutions? The only sure-fire solution I know is to find the freshly laid Monarch eggs and raise your caterpillars inside. Or if you wait long enough, perhaps until the next Monarch season, it seems like Tachinid fly problems can come and go.

July 2016 update - They're back (the terrible Tachinid flies)! I had several caterpillars turn into butterflies this spring, and since then I haven't seen many butterflies or caterpillars around. This was actually welcomed by me because it gave my backyard milkweed plants a chance to grow larger and generate new seed pods. But now I'm starting to see caterpillars on the milkweed leaves again, but all of them are ending up dead. I smashed two of the dead shriveled caterpillars I found and sure enough, the little tachinid fly maggot eggs came out. So my tachinid fly problem is back! I just had a large caterpillar I found outside turn into a chrysalis three days ago, and I crossed my fingers, but yesterday I saw the tell-tale brown spot on the outside of the chrysalis, so I know that within a few days I'll see the strings hanging from the dead chrysalis and tachinid fly maggot eggs below. It's disappointing, but it's also nature at work. At least I had some success this spring.

January 2017 update - This fall (2016) was a mixed bag. It seems to be a numbers game, i.e. if you have a LOT of caterpillars, at least a few will make it. Some of my caterpillars outside made it, but many did not. I found some empty chrysalises showing the butterflies successfully hatched, but I also found many dead caterpillars with the tell-tale thin strings dangling from them. It does seem that the Tachinid fly population fluctuates.

As for a possible solution, several readers in the comments below have recommended Rescue disposable fly traps. If you get them and try them out, please let me know how they work!


tachinid fly (larva, pupa, and fly) image gallery - right click on any image to open at full size.

(image 1) dead hanging Monarch caterpillar that failed to turn into a chrysalis. Instead, tachinid fly larvae killed it and then emerged and dropped down. Note the tell-tail dangling clear white strings where the larvae emerged. If you find this, your caterpillar was killed by tachinid flies.

(image 2) The four tachinid fly larvae that emerged from the caterpillar in image 1 and and dropped to the bottom of the glass container where they were trapped. Afer about four hours, the white tachinid fly larva will harden into reddish-tan "pods", which are the tachinid fly pupae.

(image 3) tachinid fly larvae have emerged from the now dead monarch caterpillars. Note the tell-tail clear white string trails. The larvae will wiggle around for several hours, and then harden into reddish-brown pupae. These are at the bottom of a glass jar. I pushed the wiggly larvae back next to the caterpillars to take the picture...

(image 4) tachinid fly pupae next to dead Monarch caterpillars. Two caterpillars produced thirteen (13) tachinid fly pupae.

(image 5) one of the many tachinid flies that hatched from the pupae in image 4. I kept all the pupae in a glass jar so the flies were trapped when they emerged. After all the pupae hatched into flies, I froze the jar to euthanize the flies to be able to photograph them (and keep them from getting free and killing more Monarch caterpillars).

(image 6) two of the tachinid flies from the pupae in image 4. The tachinid fly larvae emerged from the caterpillars on September 6 and "hatched" into flies on September 11, 5 days later.

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Comments / Questions / Feedback:

Comment by anndria on Friday, September 18, 2015
Hi, same problems in Pennsylvania. I raise all of mine indoors. Outside they don't have a chance around here. I lost 3 last weekend. They all make to chrysalis. The chrysalis looks healthy for a while, they it starts to discolor in an abnormal fashion. Then the next couple days the worm emerges and I have a 'bean' at the bottom of the jar. This time I am waiting for the fly to emerge. So far... nothing. 2 of them dropped 1 fly larvae, the 3rd dropped 3! Are the flies dead too? I sort of wanted to see one emerge to know what they look like. The beans have changed from a reddish brown to a blacker brown. Weird if they all die after doing all the killing. I plan to freeze and kill any that do emerge. We have released 3 out of about a dozen this year.

Reply by Steve (Cranial Borborygmus)
Anndria, Thanks for your comment. I feel your pain. My Monarch caterpillars here aren't even making it to chrysalis form. Regarding your question, after the tachinid larvae emerge from the dead caterpillar or chrysalis, they look like the white wiggly worm in my picture, and can wiggle and move quite a distance (unless they are trapped in a container). After about four hours (from what I have seen), they will harden into the reddish "bean", which is the tachinid pupa. The "bean" will get darker over time, and can eventually appear  a much darker brownish-black I think. About 5 days later, the tachinid fly will hatch and emerge from the "bean". The empty "bean" (pupa shell) can still look full, but one will see the end is open and the fly has emerged and flown away (unless you trap him).
Anyway, how many days have you been watching the "beans"? They may have not had time to hatch into flies yet. Also, are you finding eggs and bringing them in, or are you finding little caterpillars and bring them inside. In either case, but especially with little caterpillars, they may have already been injected with tachinid fly eggs.

Comment by leslie williamson on Monday, October 12, 2015
That horrible fly has gotten about 30 of my caterpillars in the last 2 weeks. I was rearing a lot of them in a screened enclosure to keep the wasps away, but then had a problem w/over crowding so let them be outside. Now this... I'm at my wits end and don't know how to keep this from happening, or how to get rid of these nasty flies. Please help.

Reply by Steve (Cranial Borborygmus)
Leslie, I feel your frustration. At this point, is seems that all of our outdoor caterpillars are being killed by Tachinid flies. I wish I know what to. I hope to find something that helps with this problem, or have someone who knows post a comment here.

Comment by Veronica on Saturday, October 24, 2015
Hi there, I think we had the same problem. My daughter found a big fat caterpillar and brought it inside and within a day it was starting to make its cocoon (I believe it was a Luna moth). Anyway it got itself all tucked away and you could still see it moving inside. We waited and waited days then weeks for it to hatch. It has been over 6 weeks and still has not hatched but about a week ago we saw a stringy thing hanging but not directly from the cocoon. Then last night my daughter saw two flies inside the container. It sounds like they were tachinid flies. I killed them but got a good look first and they had those red eyes and hardly moved that's why it was easy to kill them. I immediately went to clean out the container and found the bean looking pupae under some leaves. I moved the cocoon to a new container still attached to its stick. How do we know if the cat was infected before we captured it? The cocoon still appears to be in ok condition. There are no cracks or slits. It did seem to feel squishy and was tan but with some white discoloration. Could the cocoon have remained untouched? Not sure if we should cut open the cocoon. I'm guessing it has died since it has not hatched but could it possibly still be alive and maybe my daughter brought in infected leaves before the cat was completely enclosed in its cocoon. Any thought would be appreciated. Thanks!

Reply by Steve (Cranial Borborygmus)
Hi Veronica,
Thanks for your comment. It sounds like you had a tachinid fly problem too, and the flies probably came for the caterpillar that is in the cocoon. If this is the case and based on how long it's been, I don't think you will get a butterfly or moth. But there's no harm in continuing to wait. I would guess that the caterpillar had already been injected with the tachinid eggs when your daughter found it and brought it in. But I hope things work out. I saw a live Luna moth at a butterfly exhibit, and it was amazing.

Comment by Dena on Saturday, November 14, 2015
We had a problem with these pests with our last group of caterpillars. Lost about 3 of them; Our plants have been dormant for a couple of months. Not a bad thing they are all robust for the munching; We lost our first 3 this week; We did find an empty chrysalis. A healthy one, so i know they are not all infected; I killed a slow moving fly earlier in the week, but I am not naive enough to believe he was the only one. I really want to hang some fly paper, but i fear my monarchs will land on it and I do not want to kill them obviously; Its just so heartbreaking to watch the wonder of these creatures to have them die. Unfortunately I have not located the dropped pupae yet, I am hoping to find them before they are able to hatch. Even more discouraging is the fact that we have about 10 more caterpillars yet to pupate. Regarding the 3 caterpillars we lost this week; they all suspended on our frame we have for them-they seemed a bit small, none of them made it to their chrysalis; Today we had one go through a very "normal" process of hanging and he pupated earlier today; I am praying that he is healthy. Crossing my fingers; Any thoughts on the "fly paper"?

Reply by Steve (Cranial Borborygmus)
Dena, Thanks for your comment. I thought about fly paper also, but I think you're right that it might catch and trap some butterflies as well, so I wouldn't recommend it, and I've never tried it. I think you have touched on one answer though by finding an empty chrysalis (that would indicate at least one butterfly made it)... if you grow enough milkweed and end up with a bunch of caterpillars, hopefully at least one or two will defy the odds and will get overlooked by the Tachinid flies and survive. In the end, it may just be a numbers game. Nature can be brutal that way. But it sure is painful to find all the shriveled up dead caterpillars that don't make it. I'm still hoping to come up with some way to knock down and reduce the Tachinid fly population here.

Comment by Molly on Thursday, December 10, 2015
Learned more info from your site - so thank you! I've been raising Monarch cats and butterflies since late September when I "accidentally" stumbled upon them in my milkweed plant. I didn't know that I would find myself a new passion. My question to you is this: The tachinid fly - we have had 3 cats make it to J and Chrysalis and then succumb to tachnid fly infestation. I then started raising the cats inside in a pot that was growing milkweed. However, all the cats have now eaten all the milkweed and I've had to put them back outside where we have in-ground milkweed growing. I have seen a number of gnat like "flies" around the milkweed but not sure if it's the fly. Twice, I've seen a tachinid and killed it - i know it was tachinid because it was larger like a house fly, and slow moving and deliberate. Since you have seen the flies, probably more than me, would you know if they start out small like gnats and grow or to they come out of their pupa big -like they appear in your picture #4? (Sorry for the lengthy question).

Reply by Steve (Cranial Borborygmus)
Dena, Thanks for your comment. From my experience, the Tachinid fly is pretty large, around the size of a smaller house fly. The Tachinid flies in my pictures above are just hatched from their pupa "beans". I added the penny for reference so you can see what size the fly is when it is "born". I'm not sure if they get much bigger. The way I identify them is by their red eyes. If you are seeing small gnats, those probably aren't tachinids. Unfortunately, you may not see any around, but it only takes one showing up for a few minutes to infect a Monarch caterpillar. In my case, I raised the caterpillars in a container with a mesh over the top, and gave them fresh milkweed leaves every day. Its a bit of a pain, but it's the only sure-fire way to protect them from the tachinid flies. But in your case, the flies might be gone now, and your caterpillars may be safe "out in nature" again. Good luck!

Comment by Diane on Sunday, December 13, 2015
I'm having great success so far. I have raised 20+. Love seeing them emerge. I have been butterfly gardening about six months now. Maybe I'm just lucky? Maybe my garden is just young and the little basterds haven't found me yet? I am in Palm Beach County Florida. Ours are local rather than migrating. I have 8 hungry caterpillars eating milkweed right now. Six are big. Two are little. I had two monarchs flying about today. I'm worried about keeping enough milkweed around with leaves. I have about 7 milkweed plants in my main garden and I have started a new plot with 4 small plants. I had the pleasure of moving 3 apparent hitchhikers from the small plants to the fully grown ones.
Oh! I think I have seed pods on two of my milkweeds. I am hoping to grow more from these.
I do not know if I have seen any tachinid flies. I have seen little green flies. Are those tachnids? They don't look bristly. I did have one caterpillar die in pupa. I went to collect the pupa to inspect it after it fell but the ants must have carried it off.
I do have lots of lizards and lots of spiders. I try not to disturb every spider web in the garden as long as they are below the flower tops where butterflies visit. I have one bee as far as I can tell. It's always just one. Sad but...that's south Florida for you. I call south Florida one of the pesticide capitals of the world. Motto: If it moves, kill it.
The lizards love my little garden. I have a screened lanai. My cat loves it. I rescue lizards from her on a regular basis and put them in the garden. The lizards leave the caterpillars alone. They keep the ants down. They have plenty to eat.
Should I be killing any flies I see? What is the best way to kill them? Net?

Reply by Steve (Cranial Borborygmus)
Diane, Thank you for your EPIC comment! It's great to hear that you are having so much success, and fun! It doesn't sound like you have a Tachinid fly problem. If you did, you would be finding dead caterpillars and/or you'd find brown dead chrysalises with small holes and the little tell-tale white string where the tachinid pupa (bean) came out. I don't think the little green flies you are seeing are tachinids. Enjoy that you don't have to deal with them (at least, not yet).
As for lizards, we have a lot of blue-bellied lizards, or at least we did, until a neighborhood cat discovered it was fun to catch them. We still have quite a few. I've never seen them eat a monarch caterpillar, but the caterpillars do seem to "disappear" sometimes, so maybe they become a lizard's lunch. I love the lizards, so I don't mind if they eat a few caterpillars. I just can't stand the tachinid flies! Thanks again for your comment. Its really good to hear about the successes, and I hope you will continue to have success! 

Comment by Nancee on Saturday, December 26, 2015
Several people who have decades of experience have told me that Tachinid flies only lay their eggs ON the Monarch caterpillar.
I've always read that the fly injects the egg INTO the Monarch caterpillar.
These people live east of the Rockies. I live in southern CA. Do we have different types of Tachinid flies that lay their eggs in different ways?
I noticed that you are in Santa Barbara, and report that the T. flies inject the eggs. Is this a west coast T. fly as opposed to the eastern T. fly? Thanks

Reply by Steve (Cranial Borborygmus)
Hi Nancee, Thanks for your good question. I have read both that Tachinid flies lay eggs on caterpillars and also they inject eggs into caterpillars. I've also read they can lay eggs on or inject into the Monarch eggs. My very limited testing indicates the flies around here don't do anything to the Monarch eggs, but they do hit the very small caterpillars.
From what I have seen, I believe the fly must inject the egg into the caterpillar. I have seen a fly seem to quickly barely touch a caterpillar. The caterpillar then lets go of the leaf or stem they are on and falls to the ground. The caterpillar then flops and writhes around on the ground, as if it got bit and is in pain, and might be trying to rub something off. I don't think it would writhe (wiggle) in apparent pain like this if the egg was just placed on the outside of the caterpillar. I also don't know how the egg would stick, unless it has "glue" on it. In any event, I've never seen the actual event with enough magnification to actually see where the egg goes, but based on my observations I think the egg is "injected". I also have read there are many, many types of Tachinid flies, though I don't know how many prey on Monarchs. There could be different types in different parts of the country. BTW, I have also read that some flies lay their eggs on the plant, and then the egg (or larva?) transfers to the caterpillar when the caterpillar goes by.
Let me know if you find any more information on this question. Thanks for your comment, and for stopping by!

Comment by Jennifer on Monday, January 25, 2016
Did you ever figure out what to do about tachinid flies? We are having a real problem with them this year, eating the monarch caterpillars. We are in Chula Vista, CA & have a variety of milkweeds also - we've found the narrow leaved milkweed is a great host plant, it grows really fast & produces endless supplies of leaves. Annie's Annuals http://www.anniesannuals.com/ in Richmond, CA has a lot of milkweeds & they ship live plants.

Reply by Steve (Cranial Borborygmus)
Hi Jennifer, Unfortunately I have yet to find a very good solution to tachinid flies other than either bringing the Monarch eggs inside and raising the caterpillars where they are screened and protected from the flies, or just growing a LOT of milkweed and hoping a few of the Monarch caterpillars will get missed by the flies and will make it to becoming butterflies. Our milkweed plants here in Santa Barbara have been stripped bare for the past two months, so we really don't have any caterpillars to speak of. It looks like a few leaves are starting, so the plants may start recovering soon. Thanks for your comment and for growing milkweed. Good luck!

Comment by Erin on Friday, March 04, 2016
Ok, I have this problem big time! I had a massive monarch garden too. I am still getting butterflies because I planted so much milkweed some of the Monarchs are forming under basically a wall of milkweed where this fly seems to be struggling to access. I have lost 100's of caterpillars ahhh..

Comment by Ellen on Wednesday, March 30, 2016
Hi, same problems in Orange County CA. I started raising (indoors) monarchs last November, and have only had success bringing in FRESHLY laid eggs. We literally watch out for monarchs visiting our garden, and as soon as they leave, we collect a dozen eggs or so. With those eggs, we haven't lost a single butterfly.
So far this year (since Mid-Feb), we have released 25 monarchs, and currently have 23 chrysalides, and 22 caterpillars in different stages. My milkweed outside however is experiencing 0% success in raising caterpillars. We are constantly finding deflated cats and cats that are writhing (in pain)? Not one has made it to chrysalis form. We are finding J's with strings dangling from the tachinid fly larvae inside the cat.
It is so sad to watch these suffering cats, but we can't bring all the eggs in!

Reply by Steve (Cranial Borborygmus)
Hi Ellen, Thanks for sharing your experience. It sounds a lot like mine most of the time. I'm glad you're at least having success with bringing in freshly laid eggs. And I'm sorry to hear you're loosing all your outdoor caterpillars to the Tachinid flies. We just had our first new big crop of caterpillars for this spring. They wiped out the milkweed (it will grow back), but we ended up with a lot of big fat caterpillars and we've found several chrysalides that are all in good shape. I didn't see any flies during most of the time the cats were growing (I think I may have seen one fly at the end), so I'm hoping most of this first crop will be lucky and survive. But I'm afraid the flies will be back in force soon, and it will be another tough year. Good luck, and thanks for your efforts to help your Monarch caterpillars.

Comment by Bonnie on Thursday, May 19, 2016
I came back hoping someone had found a "cure" for those nasty flies.
They're horrible. I raise monarchs in an enclosure in my back yard in Texas. I've found rotating plants from the sun into the enclosure helps keep the plants healthy and the cats fat. Unfortunately, it also brings in cats infected by the flies. Last year I released 565 monarchs; this year, my goal is 600, and although I started out strong (212 monarchs in just the first batch), the next batch of over 100 chrysalides, started turning that nasty brown. I made the mistake of waiting until I knew for sure, for sure that the butterfly was not going to emerge, and I think that was fatal. I noticed sometimes when I'd open the enclosure flies would fly out and I wondered how they got in. Your page prompted me to open one of the chrysalides, and sure enough, maggots galore - in all of them, except those that had turned black and hung there for a while (apparently, I was breeding flies too). I cleaned out the entire batch and started fresh, and so far have gotten 7 monarchs out of about 40 chrysalides; most are still getting infected. One thing I've not seen yet is the strings hanging from the chrysalides; there are none on mine. Lots of those red-eyed pests all over the plants that are outside the enclosure, so I zap them with my electric fly-swatter. I will keep checking for a cure. Thanks for all of your helpful information! ;)

Comment by Lacey on Monday, May 23, 2016
I assume tachnid flies have been my issue too. 2015 was a major disaster but the first batch that came brought in April 2016 was a major success. At least 20 healthy butterflies hatched from my porch. Sadly this second batch has been a nightmare. I have 2 healthy chrysalis left and I'm praying they make it. A third had tachnid flies yesterday.

Comment by aggie sutton on Wednesday, June 01, 2016
Help! don't know is I have tachnid flies or not, but i may have. I live in Florida, and released about 400 monarchs last year with few fatalities. The first hatch this year was fine , but this week I wondered why so few were hatching. Well, today I removed probably 125 -50 dead chrysalis from my enclosure, I am heartbroken!
The chrysalis were mushy, black/brown, runny liquid in some and some just dry. Have not seen any strings spoken of on this site, but did see a very small fly/bug on a chrysalis, and on a cat about to go into J. Please, Please, if you have ideas for me let me know.......

Comment by Carolyn on Wednesday, July 13, 2016
Thank you so much for the wonderful info, updates and photos. After three seasons of tiring to help the monarch population by growing milkweed and collecting eggs and caterpillars "inside," this is the first summer I've lost both caterpillars and chrysalides to that #*!# fly. It breaks my heart! I am in northern Minnesota so our monarch season is very short and every caterpillar counts. Monarchwatch.org from the Univ of Kansas has an excellent lab and phenomenal website. Click on "Blog" on the home page to read Dr. Chip Taylor's July 2016 update on the plight of monarchs nationwide. It's excellent reading! Good luck the rest of the Saxon!

Comment by Irene on Wednesday, July 13, 2016
I first got two milkweed plants and was so surprised that there were soon dozens of caterpillars on them . I had to buy several more plants in order to feed them all. I tracked where they went in the garden and watched them turn into butterflies . I was fascinated / they were so beautiful. That first season I had 28 turn into butterflies. The next year I had the same result. I had to buy additional milkweed. Again they all turned into butterflies - about 35 of them. I did this same thing again and again had about :25 butterflies. They loved my yard. They flew all over. This may I had a total different horrible result. The caterpillars were leaving when they were too small and they did not turn into butterflies because that dreaded tachinidae fly ate them and I had only one that escaped. I am so sick about this . I don't understand why I never had that dreaded fly before and now he's all over all the plants. I don't know what to do . I take them down when that long string hangs down and put them in freezer to kill them.

Comment by Joann on Friday, July 15, 2016
We got our first milkweed last year in July, 2015. We saw a bumper crop of caterpillars in late Sept./Oct. and lost less than 1/2 to the tachinid fly. This year, 2016, we added 2 more milkweed plants in June and have had at least 35 caterpillars. To my knowledge, ALL have been lost to the tachinid fly. Literally no chrysalis' have successfully produced butterflies, and most caterpillars have not gotten past their "j" shape without the evil maggots making their appearances. I did begin "Hotel Caterpillar" in our house, a cardboard box with screens on 3 sides, to try to protect the smallest caterpillars. So far, complete failure. One made it to its chrysalis stage and we were so excited, but nope. Stringy disappointment.
I wonder if we had less trouble with the tachinid flies in fall 2015 because of the season? Although we are in coastal Southern California and our fall weather is quite warm, maybe the flies are less numerous due to shorter days, etc. This really is our first full summer with milkweeds. I'm interested to see if we have more success with the fall batch of caterpillars.
I haven't brought in leaves with eggs on them because I fear that when the leaf wilts, the hatched caterpillar won't be able to feed immediately and will die. Should I just supply new leaves right next to the "egg leaf"? I do want to try this option. It would be nice if my kids, and my husband and I, could get attached to a caterpillar that actually survives instead of being so disappointed every few days.
I have a thought for defending against the tachinid fly, but if this were a valid solution, wouldn't someone have posted it by now? Would a typical fly trap attract tachinid flies? It's not a pesticide, but rather a plastic bag with a powdery substance inside. Filled with water and hung in a problem area, flies are attracted to the scent, go into the bag, and get trapped. I've used one in the past in an area where flies were collecting and it was useful, but I'm not sure if tachinid flies would be attracted to the typical bait in these traps. I'll try it out since I already bought one. My thought was more tachinid flies in the bag, less on my caterpillars. Of course, they may lay their demon seed and THEN enter the bag! That would be just my luck.
Thanks, Joann

Reply by Steve (Cranial Borborygmus)
Hi Joann, Thanks for comment. Regarding bringing in the leaf with the Monarch egg on it and if the leaf wilts, I would say don't worry. In my case, I think the eggs hatched within about 3 days, and although the leaf wilts and dries a little, the tiny caterpillars can still feed on the leaf just fine. I do put fresh leaves in once I see the egg has hatched, and the caterpillars then move to the fresh leaves. So I wouldn't worry about the leaf wilting and the new caterpillar not being able to feed on the wilted leaf. They can.
I'm interested in your fly plastic bag trap. What is the powder that you bait the bag with, and how do the flies get trapped in the bag so they can't get out? If you try it on the Tachinid flies and it works, please let us know!

Comment by Brunhilde on Monday, July 25, 2016
I have a terrible grave yard of dead caterpillars on my milkweed. I would be so interested in that Fly Trap. Please keep us updated. Thank you

Comment by Lea on Tuesday, July 26, 2016
Thank you for your post. I just planted 15-20 milk weed plants in a large area of our property and have had ALL caterpillars die due to the T fly. Most make a chrylasis and then have the maggots develop inside. My kids are disappointed. Have you tried ankle bait station to try and attract them before they get big enough to mate? It is so sad to see caterpillar after caterpillar die. There has to be a way to help other than indoor raising.
Again thank you so much! At least I am not alone in this.

Comment by Bambi Avila on Sunday, July 31, 2016
My first caterpillar hung into its J position under a large milkweed leaf. I never, EVER leave the cocoons or chrysalis outside or they usually turn brown and die, and upon closer inspection they have little holes in them, which, no doubt are the result of a spider or some parasite.
Well, I removed the leaf from the plant, brought it inside, scotch-taped the leaf to a craft popsicle stick (this works) and right after it turned to a cocoon, I saw string hanging and two funny, looking seeds, or so, I thought. I flushed the "seeds" down the toilet, and an hour later there was a maggot in the bottom of the cup, and the string still hanging, so I flushed them all!
I am so sad that this has happened and is happening. I have 2 of the screened containers full of cocoons. 4 of the 9 cocoons have strings hanging down from them, but otherwise look healthy.......not brown. But I know from your posting that they must be infected. Am I going to have a deformed Monarch emerge from the cocoon? Will this affect the other cocoons? Why did my caterpillars turn all the way to cocoon before being "stricken" with this. There were no plants or flies in these habitats.
I clean them between batches, and make sure that I clean the sticky silk residue the caterpillars leave when they hang.
My next batch of caterpillars will be raised inside again they have a good, safe place in the hamper, atop and amidst their spider free pots. I check each plant thoroughly for webs, gnats, and little spiders in the dirt. I also empty out any poop in the soil and add more soil, if necessary. I then check every day for parasites I may have missed or newcomers. I have been doing this outside also, but I had no knowledge of this even fly!
Please help with more info if you can. I live on the Central Coast and we have a Eucalyptus Grove in town and we are on the migration route to Canada. My (real) name is Bambi.

Reply by Steve (Cranial Borborygmus)
Hi Bambi,
I'm sorry, but it sounds like your chrysalises are being killed by Tachinid fly maggots. They come out of the chrysalis on the little strings and will look like little wiggly white worms, then they will try to wiggle away and hide, and then a few hours later they harden into a reddish brown shell (or "seed"), which will later hatch to release the adult fly. If you are bringing in caterpillars, they may already have been "bitten" and infected by flies outside. Even little caterpillars could already have had the Tachinid eggs injected into them. There's no way of knowing until the caterpillar or chrysalis dies and the maggots emerge. I had success this spring with getting several beautiful Monarch butterflies, but now this summer the flies are back and my caterpillars and chrysalises are dying. I'm in Santa Barbara, just down the California coast from you. I wish I had a solution. The only one I can say for sure will work is if you find the monarch eggs and bring them inside before the flies can infect them. Good luck!

Comment by Sally Davis on Monday, August 08, 2016
Has anyone tried using fly strips or fly traps near their milkweed? Seems like that might help attract the flies but not the caterpillars. I hate seeing so many of my cats die.

Reply by Steve (Cranial Borborygmus)
Hi Sally,
I don't know. Several people have commented here have asked about using fly traps or fly strips, but I don't know if that would work and have never tried. I think fly strips are more for indoor use. I don't know how well they would work outside. In my backyard, I'm not sure how long they'd stay sticky (if that's how they work) because of the dust and the fog, or what else they might capture (bees, butterflies, moths, etc). If anyone has tried them and had any success, I hope they will comment here, but I suspect the fly traps/strips wouldn't work very well. Thanks for your comment, and good luck!

Comment by Meredith on Friday, August 12, 2016
I stumbled upon the "raising" of wild Monarchs by complete accident this summer after buying a different type of milkweed that I hadn't been aware of. I was so excited to find dozens of eggs all over the 6 plants I had only to start losing each and every one of them to this darn fly! Initially I didn't know it was the fly, so I started putting the caterpillars into a homemade habitat to keep them near their food source. One by one, they started dying. I read about diseases and this fly, so I gathered the nerves to squeeze one of the dead guys to check if it was in fact this fly. It was indeed the fly and that was the case for dozens of my guys! Some had two or three and some had as many as 14!!!!! I finally have 9 successful chrysalises as of today, but I can see that one has quickly turned a mucky color, which tells me it might still be infested after all. I started collecting eggs and have several who have hatched and are doing well so far. Time will tell if this does the trick for me, too. I sure hope so.
I "raised" one last year and tons as a kid, but never heard of this nasty fly. Just "glad" I am not the only one dealing with it.
Thanks for such a detailed blog about your experience!

Comment by Barry Foose on Friday, August 26, 2016
When I first planted milkweed a few years ago, I had very few problems with tachinid flies but the mortality increased during the season so I started hanging horse fly traps with bait (we have horses). I didn't see any reduction in mortality, which was 70-90%, and I read on the labels that tachinid flies were not one of the pests they would control. I talked to an Ag Dept. employee and they said they consider those flies as a beneficial insect to control caterpillars on food source plants and would never attempt to reduce their numbers.
I then started bringing in newly hatched caterpillars and raising them indoors by putting cuttings into specimen containers, containing water, with holes drilled in the cap so the cuttings would stay fresh for the babies & instars as they grew up.
This worked great and I hatched about 200 monarchs last season. It got to be a pain, however, bending down and looking for newly hatched babies so I'm now trying to bait a reusable flytrap using the remains of any infected caterpillars & chrysalis', hoping any hatched flies would release pheromones to attract new flies from outside the trap.
So far I see lots of flies in the traps but don't know if they're from the infected caterpillars I put in as bait or new flies going into the traps. The traps I'm using have a 3.5 inch wide screw off lid and meshed insert that extends when you hang it up. They're called Rescue POP Flytrap, made by Sterling International, Inc. (1-800-666-6766).
I'll know how they work in a few weeks as have started collecting all sizes of caterpillars on my plants now and the butterflies have been producing about 30-40 caterpillars per day. I mostly see large caterpillars and I've seen that some are still infected (10-20%) but am trying to keep track of the percentage but I've filled up the 6 plexiglass cages with caterpillars at various stages of growth and chrysalis' hanging from the top of the cages.
Good luck to you all. I have friends that say they've had 99-100% mortality so hope this works.

Reply by Steve (Cranial Borborygmus)
Hi Barry,
Thanks for the great comment and information. Please report back on how your flytraps work.

Comment by Deedee Forrester on Friday, September 09, 2016
Same info as above commenters. I didn't know what was killing my wonderful monarchs, but all the caterpillars or chrysalis that have died had a white string hanging from them. Now I know! The T fly! I only have 8 milkweeds and have been raising monarchs for over a year now. But the past few months have been dreadful. Not sure what to do. Either get rid of all my plants and start over or try the fly traps. I will try the flytraps first. I just feel like I have the milkweed to attract monarchs just to feed and breed the damn T flies!

Comment by Kim on Wednesday, September 14, 2016
Inland San Diego - I've been using the Rescue Fly Traps for many years because the T Fly infects 100% of my caterpillars. The only good fly is a dead fly! Lizards were also getting them. The only way I can have success is to bring eggs in or the teeny tiny newborns - instage 1 or 2 but not over that age. It's too late then.
The Rescue Fly Traps (POP is in a recycled soda bottle) and I think it smells a little milder than the regular Rescue ones. I love seeing them fill up and I will use them year-round! I had them above my milkweed but the area filled up with flies so that's probably a bad idea so I moved them to the edge of our backyard.

Comment by Amanda on Wednesday, September 28, 2016
My son and I were checking on our caterpillars when all of a sudden one fell. I just thought it was a fluke, but it was twitching all over. Then 2 more ended up falling and doing the same thing. Then I noticed that a fly was around. So I picked up the 3 caterpillars and put them in a cage to see if they have been infected.
I stumbled across this blog, and now I'm sure of what happened! Sad face.
We live in College Station, Texas and right now we have over 20 on one of our volunteer plants! We have NEVER had this many at once. So I'm hoping they all make it. My husband is an elementary school teacher and is bringing me home more cages!

Reply by Steve (Cranial Borborygmus)
Hi Amanda, Unfortunately, you saw and described exactly what happens when a Tachinid fly quickly lands on a Monarch caterpillar and injects (or tries to inject) a fly egg into the caterpillar. It literally happens in the blink of an eye. The caterpillar will then let go and drop to the ground, and then flip and twist all over the place. I've seen it happen too many times, and now that I know what it means, it makes me very sad. I once saw a caterpillar drop onto the ground. I then spotted the fly, which continued to "dive bomb" the twitching caterpillar on the ground until I moved the caterpillar and tried to shoe the fly away (I wasn't able to kill it). I don't know if that caterpillar survived. As for your case, I don't know if the fly was successful in injecting a fly egg, but I hope not, and I hope that the caterpillars that you rescued will be ok.

Comment by Anita on Friday, September 30, 2016
I have been growing a variety of milkweed plants (mostly native) in my Southern California backyard for about three years now and have NEVER seen the destruction by the t-flies that I have seen this year. My first spring batch of caterpillars grew plump and healthy and we had great success all the way to butterfly stage. After that, the flies have taken almost 100% of my cats. I made a large screen "cage" out of PVC pipe and netting that I placed around some of my plants, thinking that this would keep the flies out, but they somehow seemed to find a way in. I've lost dozens and dozens of caterpillars. It is frustrating and so sad to watch the beautiful cats shrink and die, especially after making it all the way to the "J" form. I'm about to give up. I will be trying some of the suggestions I have read on your site. Thank you! I'll continue searching for answers until I decide to totally surrender...

Comment by Liz in Louisiana on Friday, September 30, 2016
I feel I am almost ready to cry after reading all the disappointing news about that t-fly. I too have been growing milkweed for several years and this year have had an abundance of cat. I counted in excess of 20 one afternoon, the next day they were all gone. I have read about the fly and did notice one that same day. I decided to try "growing my own" started with two, ate furiously and made it to the chrysalis stage, but tonight when checking them i found the white strands coming from one, still green I know I need to destroy it and I am heartbroken. The other seems fine, at least today. Someone push me in the right direction as to the infected one. Should I just separate them or just go ahead and freeze it?

Reply by Steve (Cranial Borborygmus)
Hi Liz, If a chrysalis develops a brown spot on it, that means it has a tachinid fly maggot inside it and the caterpillar has died or will die. When you see the brown spot, you should probably freeze the chrysalis before the maggot emerges. Once you see the white thread, that means the maggot has already come out, but there may be more maggots that will continue to emerge. So if you see the white thread, the chrysalis is dead and should be euthanized. I know it is sad, and it breaks my heart as well. As for your other chrysalis that is still all green and looks ok, it could be ok and healthy. Just because one caterpillar got infected doesn't mean the other one did, though it might have. I would keep an eye on the healthy chrysalis and cross your fingers. Good luck. 

Comment by Hello on Sunday, November 06, 2016
I'm 10 and I'm scared for my cats. I have read that the t fly kills eggs and it happened to me. I saw a teachings fly jus laying eggs in my monarch egg. I do not now why it's happening to me but not to you, but please help me for my only and first cat, and be careful.

Reply by Steve (Cranial Borborygmus)
Thanks for your comment. I think you may be my youngest reader! I'm very happy to hear that kids are involved in helping the Monarch butterfly! I don't think that the tachinid fly will actually lay its eggs inside a Monarch egg, but I don't know for sure. I've had good success when I've found and brought Monarch eggs inside, and have let them hatch so that the (indoor) caterpillars are then protected from the tachinid flies. Maybe the fly you saw just walked over the Monarch egg, but didn't actually do anything. I guess you won't know until the caterpillar hatches and grows a bit. Good luck. I've got my fingers crossed for your cats!

Comment by Jim Morrow on Sunday, November 06, 2016
In order to solve the Tachinid problem, I bought some disposable, Rescue (brand name) fly traps via Amazon. It's a plastic bag, with an attractant already inside. Just hang it up and add water. Don't add water first, as they get heavy and un-wieldy. It definitely catches flies--it also has an odor-- hang them away from your home. I have examined some of the dead flies, but cannot tell if any Tachinid flies are being caught. If you are so inclined, these plastic bags can be cleaned out, refilled, and used again. I now have about 12 chrysalis, and so far have lost 3 to the fly. I put a cardboard container underneath the chrysalis, catch the fly larva after they come out and eliminate them. Rescue makes a re-usable trap (plastic), that I bought @ Armstrong Nursery and also Amazon. They're about $ 25.00 each. They are much more successful at catching flies. I re-use the strained attractant water, and add a bunch of smelly fish parts. Definitely great at attracting/eliminating flies. I have to believe there has to be some way to attract these flies and catch them.

Reply by Steve (Cranial Borborygmus)
Hi Jim, Thanks for your comment and info. It sounds like you have a good system for catching flies, but you're not sure if you're catching regular flies or tachinid flies, or both. I did a quick search on Amazon and based on your description found this two pack of Rescue disposable fly traps for $11.74. I like that the traps are pesticide-free, and they can catch up to 20,000 flies (that's a LOT of flies). It looks like they have a lot of positive reviews on the Amazon product page. Definitely worth looking into. Thanks.

Comment by Bloomingdalekid on Tuesday, December 06, 2016
I have successfully released 85 monarch butterflies this season. Having said that, I lost more than 25 to the tachnid fly before I purchased an indoor cage to raise them in. I would go hunting for eggs on a daily basis and watch them emerge into a beautiful monarch. It was great fun releasing them and watching them fly away. I am looking forward to a good season in 2017 - it's miraculous to me to watch them. Nature at its best. Those Tachnid flies are terrible. I actually euthanize caterpillars outside if they get larger than "just hatched" as I know they will die, and the result is 5 more tachnids per dead caterpillar. My backyard was just covered with them this year - and I don't know of any solutions to getting them to go to your neighbors house..... if you get my drift.

Comment by Betty Arrington, Jupiter, Fl. on Monday, January 02, 2017
There is a parasitic wasp the size of a fruit gnat. I don't know if this tachinid fly is the same thing. The government was releasing a tiny parasitic wasp to so call control an insect they said was causing canker in citrus. They've released so many harmful insects for one excuse or another. That I don't think butterflies have much of a future the way they have declined in high numbers in just a few years.

Reply by Steve (Cranial Borborygmus)
Hi Betty, Thanks for your comment. The parasitic wasp is different from the Tachinid fly, but it might also be a problem for the Monarch butterfly. But you raise an interesting point. When I was researching "tachinid fly control" I discovered that a lot of people (i.e. farmers) where not trying to control the tachinid fly population, but instead they were using tachinid flies to control the population of caterpillars. Not Monarch caterpillars, but other "pest" caterpillars that can eat a lot of valuable crops (and thus are considered "pests"). Pest insects are a real issue, as I know from being a backyard farmer. Slugs and snails regularly decimate my green bean seedlings! One just has to balance attempting to get rid of harmful insects with not inadvertently getting rid of beneficial insects, like the Monarch butterfly.

Comment by Beth on Tuesday, February 28, 2017
I am in Texas and lost a number of Monarchs to the Tachnid fly last year -- and maybe more than I thought. I blamed a number of others on the OE parasite or bacterial infection. I counted 34 eggs this morning -- all laid in the last few days. We saw a butterfly or two over the weekend. Last year, I waited and brought in the hatched cats, but I think I will start moving the eggs this year. And buy some of those fly traps. I am already seeing small flies on and around my olants. I tried the yellow tape kind last year and they didn't seem to do much. If you are having cats disappear, I have had both red wasps and anole (the small green chameleon lizards) go after the more mature cats. The wasps will go after the instar 4-5 cats and come back for more. Really made me mad after buying more plants to feed a large group of cats. That's when I started my enclosure.

Reply by Steve (Cranial Borborygmus)
Hi Beth, Thanks for your comment. I hope you have better luck this year. Thanks for growing milkweed and supporting the Monarch butterfly!

Comment by Linda on the Texas Gulf Coast on Wednesday, March 01, 2017
I have been raising Monarchs for several years and only recently experienced the attack of the Tachnid flies. I have had some success using the Rescue disposable fly traps. You can buy them at Home Depot. My son built me a rearing cage using PVC pipe and pliable screening. I also am trying Agfabric that I bought on Amazon to cover my potted plants once I see the butterflies lay eggs on my milkweed. The Agfabric lets in sunshine and you can water your plants without having to remove it. I also use the Agfabric to give my milkweed a chance to recover and grow new leaves. This combination seem to be working fairly well. Most of these preventative measures have been implemented only recently, but I am already seeing an improvement in the number of emerging Monarchs. I hope you find this information beneficial.

Reply by Steve (Cranial Borborygmus)
Hi l.inda, Thank you very much for commenting and sharing your experience and advice!

Comment by Ellen on Saturday, March 18, 2017
Hi! We raise Monarch butterflies in Victoria, southern Australia, and Tachinid flies are a pest here too! I have asked other people in the area to collect their caterpillars and raise them inside, but they ignore my advice......and their plants are covered in pupae that have been parasitised. As they don't collect the ruined pupae, the Tachinid fly maggots drop to the ground, and so can parasitise more poor unsuspecting Monarchs. It is such a waste of life for the Monarchs.

Reply by Steve (Cranial Borborygmus)
Hi Ellen, Thank you commenting! I can understand your frustration. It's unfortunate that other people near you don't understand and/or don't care enough about what's going on. You are right that the Tachinid flies that breed and multiply in one place can then spread. It is a tough fight, and it would help if more people participated. Thanks for doing what you are doing to help the Monarch butterfly!

Comment by Linda Vincent on Wednesday, April 12, 2017
Those flies you are talking about are laying eggs on all my butterfly bushes. They lay little black eggs that look like large pepper. Then the leaf gets deformed, and it becomes a maggot. I city the plants down to nothing. When the plants started leafing out I sprayed with soap spray. Before I sprayed I did see some of those flies and smushed them. I may have to get rid of all the bushes or keep them in an area away from other plants. My whit bush is ten years old gets ten feet tall and is beautiful.

Reply by Steve (Cranial Borborygmus)
Hi Linda, I think you are dealing with something else. The Tachinid fly lays its eggs on or injects them into the Monarch caterpillar. The tachinid maggot then hatches and lives in and feeds on the caterpillar (the maggot is  a parasite). It eventually kills the caterpillar or chrysalis, and emerges and turns into a little brownish-red bean, then hatches again to become the adult fly. It does not feed on plant leaves or effect the plant. If you are getting black spots on your leaves that are killing the leaves, that is something else, though it sounds equally frustrating. Good luck!

Comment by Reek Havok on Thursday, April 27, 2017
Hi, Great post and thanks for sharing. I have found out the exact same thing you have, that you have to quarantine the caterpillars or eggs if you can get them before they hatch. Unfortunately, I had 17 caterpillars that all dies at once. I was running out of Milkweed so I went to the nursery and dropped another $55 on 3 plants, two small and one large. Something was on those plants as the next day 12 of my caterpillars were dead and over the next 3 days the others also died. I have a large netted habitat I keep them in and have had three successful Monarchs this year and 6 more healthy looking Chrysalies.

Comment by Leigh-Ann on Tuesday, May 02, 2017
Hi there, so we have had a few monarch caterpillars which we have raised in captivity. Always had no issues but lately a lot have them have died ours and the ones we get them from in the wild (my child's preschool has a big milkweed tree which we get ours from as my 3 girls love watching the process and then releasing the butterflies). They have always died while still caterpillars and as soon as they die (about 3 of them so far in captivity) I throw them away outside so I just always assumed they were not good due to how many there have been lately and I've seen so many die at the school in the wild I thought it was mother nature just culling. Well I got another one about 1/2 inch long, picked a fresh leaf for it daily and it was doing great, ate a lot got really big and became a chrysalis 2 days ago. It was looking so good until this morning (day 2) the chrysalis looked sunken in, half splitting at the top with this white string stuff hanging from it. When I looked at it there were these 2 little reddish brown hard seed looking things at the bottom of the cage. I cleaned the cage out and threw the seed things away (wish I had taken a pic) and then did a little google research and found your page and your pics, so the red brown "seeds" were exactly like the ones in your pic so now I know it was due to these flies. I did take a pic of the chrysalis before I trashed it though. I have another 2 much smaller caterpillars still so I'm hoping these little ones are ok. Please let me know if you want the pic I took of the chrysalis. Ps. We are in Honolulu, Hawaii.

Reply by Steve (Cranial Borborygmus)
Hi Leigh-Ann, Thanks for commenting! I did not know there were Monarch butterflies in Hawaii. How cool! Based on your description of the white string and two reddish brown beans, you also have the Tachinid flies, and they are killing the caterpillars and chrysalises. If you find any more "beans" please smash them before you discard them, or they'll hatch later into another fly. I hope your new caterpillars make it, and I'm glad your girls get to see the caterpillar to chrysalis to butterfly process. It's a wonderful way to experience part of nature!

Comment by Tom San Diego CA on Thursday, May 11, 2017
I got started this year growing milkweed and raising cats. Had some early success. Now get cats to a J under a milkweed leaf, but then don't seem to have the strength to form beyond that. I have seen the silk strands from the dead cats and even found a maggot worm on the bottom of my 2' x 4' screened pen. Very disappointing. As a beginner, I can only add the following as a possible additional way they infect. I read the tachinid flys lay eggs on leaves, the leaves get eaten then ingested by the host cat.

Reply by Steve (Cranial Borborygmus)
Hi Tom, Since you are finding the white strings, you are dealing with Tachinid flies killing your caterpillars. I understand how frustrating it can be! Thanks for providing the link to the very interesting article. There are lots of types of tachinid flies (around 1300 different species according to your article). As you mention, some can lay their eggs on leaves and hope the insect ingests the eggs. But all of my research indicates this does not happen with the kind of tachinid fly that preys on Monarch caterpillars. In fact, from your article, it states: "The tachinid species that seek out moth and butterfly hosts usually glue their eggs to the moth or butterfly larvae; after the eggs hatch, the resulting fly larvae bore into their host, eventually killing it." I also find the article interesting because it make the point that many consider the tachinid fly to be a "beneficial" insect because they kill "pest" insects! Unfortunately, the flies also target Monarch caterpillars. In any event, keep growing milkweed! It can be frustrating, but from my experience, the tachinid fly problem can come and go, and you should get at least some Monarch caterpillars surviving and becoming butterflies. Good luck, and thank for commenting!

Comment by Paula on Wednesday, May 24, 2017
Pest problems are often made much worse when a monoculture is planted. Has anyone tried sticking to smaller sized milkweed farms? Would this help?

Reply by Steve (Cranial Borborygmus)
Hi Paula, Thanks for your comment. I think the monoculture issue probably replies to larger farms than the amount of milkweed that we "backyard farmers" grow. That said, it would be a catch-22 situation. If one grew less milkweed and had fewer caterpillars, there might be a lower risk of Tachinid flies showing up, but if the flies show up, you'd have less caterpillars and the odds of any of them surviving would be less. Moreover, I think most of us who grow milkweed try to grow as much as possible, since it is amazing how much milkweed hungry caterpillars can eat, and it's heartbreaking to run out of milkweed when you still have caterpillars around. But if one does have room to grow a lot of milkweed, it's probably a good idea to have a few different types, as well as having other nectar-bearing flowers for the adult butterflies. Thanks again for commenting!

Comment by Christy on Monday, May 29, 2017
WANT TO AVOID TACHINID FLIES IN THE GARDEN? I pulled this information off of a university website. If we watch the companion plantings, it may deter tachinids from visiting as frequently. Release some ladybugs just at sunset after watering the garden...they are more likely to stay and then eat up aphids which also attract tachinids.
"Most adult tachinid flies feed on nectar and pollen, especially from flowering umbelliferous plants such as carrot, dill, fennel, queen anne's lace, and other herbs and composite flowers such as asters and rudbeckias, as well as other flowering plants. They will also feed on aphid honeydew, so having non-crop plants infested with aphids helps support tachinid flies."

Reply by Steve (Cranial Borborygmus)
Hi Christy, Thanks for your comment and info. Very interesting... There are a lot of different species of Tachinid flies, so I'm not sure if your info applies to the flies that prey on Monarch caterpillars. Further, I don't have any of the plants mentioned in my yard, but I still get Tachinid fly problems. I also wouldn't want to remove all the flowers since the Monarch butterflies and bees need them. I do get the milkweed aphid infestations (which are hard on the milkweed plants) and it would be interesting if the aphids then also attract the tachinid fly. It would be nice to get rid of the aphids.

Comment by Kevin on Tuesday, May 30, 2017
I have grown milkweed for a good ten years plus in Southern California. But only the orange and yellow flowered milkweed. I have never had a problem with trachind flies. This past year I purchased a milkweed plant with only yellow flowers. I now seem to have a issue with trachnide flies. Is there some type of relation to the type of milkweed plant that I'm growing?

Reply by Steve (Cranial Borborygmus)
Hi Kevin, Thanks for your comment. Sounds like you had great luck for ten years! I don't know if the tachinid fly problem is related to the type of milkweed grow. I don't think so. I grow "tropical" milkweed, but I know other people with "regular" and/or native milkweed, and they also have the tachinid fly problem. At least in my case, the problem comes and goes, and this spring it hasn't been too bad here.

Comment by Walter on Saturday, June 10, 2017
Hello brothers and sisters - I look forward to someone solving the tachinid fly problem. But in the meantime, I can share with you what I do. It has about 60 to 70% success rate. I made a large pen about 4 feet high by 3 feet wide by 3 feet deep of PVC and attached screening to it using short pan head screws. Inside that pen, which is very light and easily lifts off, are 8 pots of milkweed plants. My normal garden sprinklers keep them watered through the screen. Then in another area of the yard I have three pots that are in the ground. They're in the ground, but they are removable. The visiting butterflies lay eggs on the three pots that are in the ground. Every other day or so, as soon as I see a caterpillar, I transport him to the safe caged area. I simply lift the cage off, attend to the caterpillars and replace the cage. Eventually, the caterpillars will consume most of the plants inside the caged area. That's when I start swapping them out for the pots in the other part of the yard that are in the ground. It gives them a chance to regrow, and I just keep moving the plants back-and-forth. For me this is considerably less work than bringing them indoors. Also the mature caterpillars will climb to the top of the safe cage and create their chrysalides. The can be collected and hung from a little twig stand indoors until they emerge. I've given away twig stands with chrysalides to friends and teachers. When it's time for them to emerge I put the twig stand outside with a mini version of the safe cage 12" x 12" x 12" and just lift it up when the butterfly is ready to fly away. Cost of the safe cages is just a few dollars. I might make a walk in cage this season.

Reply by Steve (Cranial Borborygmus)
Hi Walter, Wow. You have a cool system in place. Thanks for sharing!

Comment by Robert Burns on Saturday, June 17, 2017
The damned fly lays its eggs on larvae not eggs. There are parasitic wasps that infect butterfly eggs but, if they target Monarch eggs, they are not nearly the threat of this despicable fly.

Reply by Steve (Cranial Borborygmus)
Hi Robert, Thanks for your comment. Although some other readers here have said they think the Tachinid fly might lay its eggs on the Monarch eggs, I agree with you. My understanding and experience is that the Tachinid fly only lays its own eggs on the Monarch caterpillars, and not on the eggs. When I've collected Monarch eggs and brought them inside to hatch, the caterpillars have done just fine. And I agree with you that the Tachinid fly is a damned, despicable fly!

Comment by BLT on Monday, June 19, 2017
Anxiously awaiting any / all updates on success, or not, with the fly traps! I had one (known) successful Monarch 'birth' this Spring, but since none. All of my plants are outside (in Santa Monica) so can't keep track of all my caterpillars - have found some dead in the garden, some hanging as string-laden chrysalis and the others just wander off and never call or text. Thank you for this site and all your information!

Comment by Robert Burns on Tuesday, June 27, 2017
When I was a kid living for weeks at a time with my grandparents on their farms in Ohio I regularly collected Monarch caterpillars and had my share of infected ones. However, I searched for them (and berries) so regularly that I would find the caterpillars with eggs attached to them which hadn't had a chance to hatch and squished the eggs with tweezers with the caterpillars surviving. My system was a little like mosquito control by use of sterile males. I now understand that there is a counter-parasitic wasp whose larvae heroically attack parasitic larvae within caterpillars apparently allowing the caterpillars to survive. (https://en.wikipedia.org/wiki/Trigonalidae)
More of these may be needed. Hell is paved with parasites.

Comment by Jane Lerche on Friday, June 30, 2017
My hubby and I love the monarch butterflies. We live in Texas. Last year we had monarchs eating our milkweed from early spring to the first part of January when the last one we watched, flew away. This year, they came in early spring, are still flying around all the time, but not laying eggs....nothing eating our milkweed. Do you know what is so different about 2017?

Reply by Steve (Cranial Borborygmus)
Hi Jane, I'm actually seeing something similar here in coastal Southern California this year. Usually all of our milkweed would be wiped out by now, but we haven't seen many caterpillars or butterflies (or tachinid flies), and for a change, we have plenty of milkweed (and I keep planting more). I don't know why there aren't many caterpillars around right now where you are or here, but I think it's just a coincidence. I think nature just naturally ebbs and flows. I'm sure lots of hungry caterpillars will show up eventually! Thanks for commenting!

Reply by Jane (Cranial Borborygmus)
Hope you are right that it is just a coincidence. I'd sure hate see the beautiful monarchs disappear! I let the milkweed just grow wherever if came up this year, so I have it everywhere and no caterpillars!

Comment by mark gerrie on Sunday, July 09, 2017
Helpful comments above confirms my observations. I have about ten potted milkweed from which batches of monarch eggs are hatched. Many cats go missing but witnessed what looked like a no male fly swoop down a simply "tag" a large cat who then dropped and withered as though stung. This cat no doubt won't make it, but I have since clustered the potted milkweed and fashioned a screen enclosure as a barrier against further attacks. Those cats gone missing are apparantly wandering off to shed, then can't find the potted milkweed again. Hoping the screen enclosure helps combat this as well. Biggest problem is OE, with no resolution. So. Cal. Mark

Reply by Steve (Cranial Borborygmus)
Hi Mark, Your description of a fly swooping in and "tagging" the large caterpillar, and then the caterpillar dropping and withering as though strung, is exactly what I've seen and is what happens when the tachinid fly lays its egg on/in the caterpillar. I hope your screen enclosure helps. Good luck!

Comment by Amy Gallagher on Tuesday, July 11, 2017
I ordered two monarch larva online with 4 potted milkweed plants and a netted habitat. When I received them, everything looked good. The next day I could not find the caterpillars in the habitat. The day after that i noticed little gnats in the habitat. Could these "gnats" be Tachinid flies?

Reply by Steve (Cranial Borborygmus)
Hi Amy, I don't know, but that seems quick. Usually if the caterpillar was infected with the fly parasite, the tachinid maggot kills the caterpillar or chrysalis and then emerges leaving a little white string. Within hours the white maggot changes to a little brownish red hard egg shape. After a few days, the fly hatches and emerges.
By the way, where did you order milkweed plants and/or caterpillars online, and how were they shipped to you?

Reply by Amy Gallagher on Wednesday, July 12, 2017
They were shipped over night. I think the name of the website is Shady Oaks Butterfly Farm. If the caterpillars died, would they curl up into a tiny ball that would be hard to see? Thank you! Amy

Reply by Steve (Cranial Borborygmus)
Hi Amy, If the caterpillars were killed by the tachinid maggot/parasite, you would still find the caterpillar. It doesn't curl up, it just dies. If you haven't found the caterpillars, they may have wandered off to pupate (turn into a chrysalis). They usually don't stay on the milkweed plant when they are ready to pupate, and instead they can wander a few feet or up to 50 to 100 feet looking for a safe place to hang and pupate.
You might want to email the seller and get their opinion on what happened. Good luck!

Comment by Marge Emerson on Wednesday, July 12, 2017
Thanks for your very helpful and interesting post. I'm in Oak View, CA - just lost another caterpillar to tachnid fly - this one was hanging from the cage ceiling. I saw the 2 white threads - quickly removed the caterpillar and collected 3 maggots from the floor below - didn't dare disinfect area as the other caterpillars and chrysalises (8) seem fine. Earlier, when one chrysalis outside dropped it's 'thread' - I had to look up what had happened - 'didn't know about tachnid fly then. I've since removed some of the earth below where the one chrysalis died and hopefully disinfected the soil with hydrogen peroxide and water. I've not been successful raising monarch from eggs as yet. Face that there are half grown caterpillars to bring to the cage, means there must be eggs - I'm not good at finding then and disinfecting them with bleach and water (1/20) for 60 seconds. This is my first year at this - early in the year all 3 chrysalises died of OE. I will look up the life cycle of tachnid fly to see if the recent dead one was infected before or after my attempt at sterilizing the floor - 13 days before this happened. Recent one that died was from the same outside area.

Reply by Steve (Cranial Borborygmus)
Hi Marge, You don't have to disinfect anything to stop the tachinid fly. The only way a caterpillar is "infected" is by an adult fly, and it happens when the adult fly lands on the caterpillar and attaches or injects its egg into the caterpillar. You just need to keep the flies away from the caterpillars, which can be tough. Also, if you already have tachinid problems, it helps to destroy the dead caterpillars and chrysalises before the tachinid maggots emerge, or if the maggots have emerged, then find and destroy the maggots or little brownish red bean-shaped eggs that they turn into before they hatch into flies (see my pictures). Finding the little brownish red bean-shaped eggs is almost impossible though. So don't worry about disinfecting anything with regard to the tachinid fly. OE is different and probably does require disinfecting, but I haven't had to deal with OE yet (fingers crossed). Good luck!

Reply by Marge Emerson on Wednesday, July 12, 2017
Thanks for prompt reply. I was told I need to disinfect eggs for OE - also was shown importance of cutting down the tropical (non native) milkweed each season and learned how to disinfect the cuttings I saved and am now growing in pots - hopefully getting OE free monarchs. I just read more about tachinid - learned they are used by ag in biological pest control species as they kill grass hoppers and Japanese beetles. Monarchs have many 'challenges.' M

Comment by Kimberly Tyler on Thursday, July 20, 2017
Two comments... someone mentioned buying milkweed to feed cats. Many growers put insecticides on plants so nothing (cats) eat them so they look nice when you buy them. Ask the nursery but try to keep butterflies away for two weeks+.
I bring in leaves with eggs, rinse and rub the non-egg areas then moisten a bit of paper towel, wrap it around the base of the leaf and wrap a bit of plastic wrap around that. As soon as the egg hatches I cut the leaf around the cat and place it on a fresh leaf.
I keep the eggs and small cats (until about 1/2") in sealable glass containers on top of a paper towel in the bottom so I can see them. Once they grow larger I put them in glass jars with a paper towel circle on the floor and cover the top with plastic netting I get by taking apart scrunchy wash puff balls.
I would love for people who are raising lots inside (especially the 500+ per year!) to email me directly to show me the containers they use, how they shelve them, etc. Right now my 46 eggs and cats are taking up my dining room!
Thanks! Kimberly...   kjkooyenga <at> cs.com

Reply by Steve (Cranial Borborygmus)
Hi Kimberly, Thanks for your comment. I have heard that some nurseries might sell milkweed plants that have been sprayed with insecticide, which is just plain crazy (and stupid) since the only reason I can see people buying milkweed plants is so that monarch caterpillars can feed on them. I guess one needs to be aware of this, and ask your local nursery to confirm their milkweed plants are insecticide-free before you buy them.
Thanks also for sharing your system for how you are raising your caterpillars. I have included your email address as you requested if other readers want to email you directly.

Comment by Cindee on Saturday, July 22, 2017
I'm so glad I found this website, especially after reading posts from my little town of Oak View California. This is my 2nd year raising monarchs. My 1st year I had so many I lost count but this summer I've also been hit hard by the tachinid fly. This morning my last & only out of over 20 crysalis's (sp?) became a beautiful butterfly. I purchased a screened cage thingy & am now putting eggs in it although they are not just laid. Will keep you updated.

Comment by Roxanne Schorbach on Sunday, July 23, 2017
THANK you for your website and the information posted. My husband and I have been raising monarchs for 3 years and this seems to be the first year we have had a problem with the Tachinid fly. We were about to give up after losing our last 6 cats not understanding what was happening. I took out my macro lens and took photos of one of the cats yesterday, then some video and that's when we noticed the cat moving, then the insides bulging. When viewed on screen we could see something inside squirming around. It was like a horror movie. I just finished reading all of your posts and now understand what's been happening so don't plan to give up. Fortunately, we found one of our milkweeds with several eggs on it and brought it into the patio. The eggs have since hatched. We have them in what we call our "Cat House #2" so they have been protected from any flies. I too would love to see these little guys grow naturally in the garden but will keep them inside until, hopefully, they eventually emerge as monarchs. This will be our first time raising monarchs from eggs. Regarding the fly trap, I plan to hang a couple in my patio . . . The several monarchs that we have in our yard, front and back, don't seem to come into the patio area. We also put a rotating fan in our patio which helps to discourage flies. This is where we keep our two "Cat Houses." Thank you again for all the information you have posted - please keep it going for future monarch enthusiasts.

Reply by Steve (Cranial Borborygmus)
Hi Roxanne, Thanks for your comment. It's amazing that you were able to make a video of what appears to be the Tachinid fly parasite/maggot moving around inside a Monarch caterpillar. I'd be tempted to want to watch it, but at the same time I'm afraid it would be too creepy and disturbing, and I wouldn't be able to get the image out of my head. I'm pretty squeamish about things like that, and I also don't like horror movies! Anyway, it's always great to hear from another Monarch enthusiast, and it's fantastic what you're doing to help the Monarch!

Comment by Sandy Anderson on Wednesday, July 26, 2017
Well I have just discovered the horrible tachinid fly. In my second year of raising butterflies, I currently raise Gulf Fritillary, Eastern Black Swallowtail, Pologamas, Zebra Longwing and of course Monarchs. I've been successful in keeping the milkweed bugs, aphids, wasps and assassin bugs at bay, however I've just lost the last 3 caterpillars to this fly I looked at as harmless. I noted them flying around the milkweed but was not educated enough to panic.
As of today I have released 1,245 Monarchs. All raised outside, I collect the caterpillars, house them in a screened holding area on their plants. When they are within a day or two of going to chrysalis I separate them and put them in their own containers. Once they are chrysalis I transfer to a safe house and when ready to emerge to a flight cage. So the process is as predator free.
I have over 200 milkweed plants, some planted, some in pots. Other than building a screened room I don't see how to keep them safe from the flies. I am going to buy a shop vac and outfit it with a long tube to hopefully suck up the little murders. Does anyone who what their natural predators are?
Thank you for the information. And format to discuss our heartbreak!

Reply by Steve (Cranial Borborygmus)
Hi Sandy, Thank you for your comment and your efforts for the Monarch and the other butterflies. It is incredible and fantastic that you have released 1,245 Monarch butterflies. Wow!

Comment by Sandra Anderson on Thursday, July 27, 2017
Ok, folks...I tried the shop vac, failure, could not get within 6 inches of them. Collected about 15 in plastic containers. They are going to die! After 3 hours of policing my yard I made a great discovery....they do not see the color white. With paper towels over my palms I was able to smash them as they searched my milkweed. Today I got 10 that way. Not sure how far you all want to go with this problem, but give "white" a try. I also noted they are on my white plastic table in the early morning. Go get them!

Comment by Heidi on Thursday, July 27, 2017
I just started having this problem. One thing I noticed was a praying mantis in my milkweed eating a fly. So I'm going to try buying some of them.

Comment by Rob Wood on Friday, July 28, 2017
Thanks for addressing this incredibly frustrating and discouraging problem! We have a butterfly garden in our front yard in North San Diego county, with lush, second-year growth native California milkweed, as well as very healthy tropical weed, and no shortage of caterpillars. I don't raise caterpillars indoors - yet - as I really want to live in harmony with Nature, and not artificially circumvent the natural way of things. That said, we are currently plagued with the Tachinid fly problem, and so far this year, I'm guessing we've had a total of 2 or 3 caterpillars (out of 100 at least) emerge as butterflies. All the rest have been killed by predators, and in the last week, 100% of our 5th Instars have been killed by Tachinids. I'm hoping there is a natural plant that repels them, similar to they way marigolds repel some pests. Has anyone had any success going this route?

Reply by Steve (Cranial Borborygmus)
Hi Rob, Unfortunately, I'm not aware of any plants that repel the Tachinid fly, though it is a neat idea. It's interesting that you're in North San Diego county and are reporting lots of caterpillars this year. I'm essentially just up the coast a bit in Santa Barbara, and so far this year we had a lot fewer butterflies and caterpillars than in the past. On the positive side, our milkweed is doing great and we're growing quite a supply. Maybe you can tell some of your butterflies to head north. :) Anyway, good luck. I agree with you on wanting to live in harmony with nature and on keeping things as "natural" as possible, but seeing so many caterpillars killed by Tachinids is tough. Thanks for commenting.

Comment by Rob Wood on Friday, July 28, 2017
I've been wrestling with this problem for a while now, and I've come to the conclusion that the best way to help the Monarchs is to restore their native food supply. I love our little butterfly garden, don't get me wrong, but if we were all to put the same money and time we're devoting to battling Tachinids and OE into convincing our local and state leaders to plant milkweed in appropriate environments, the Monarch numbers would be back where they need to be in no time.

Comment by Rob Wood on Friday, July 28, 2017
On advice from the Monarch Joint Venture folks, this morning I separated our two large milkweed planters across the yard from each other. According to MJV, concentrating caterpillars attracts more predators, which makes a lot of sense. We'll see if that makes a difference.
On another subject: tropicals. It seems to me that our tachninid problem began to get out of hand as we added a planter full of tropical Butterfly weed. They are full of nectar-producing flowers, and I've read that tachinids are attracted to nectar. I'm wondering if anyone in this forum has set up a garden limited to native California, narrow leaf plants, and if you've noticed a difference.

Comment by Rob Wood on Saturday, July 29, 2017
3 nights ago, I had 6 healthy-looking 5th Instars left in my garden. Today, I found all 6 dead, hanging upside down in pre-chrysalis position, with tachinid strings streaming from their bodies. That's a 100% mortality rate for this generation of caterpillars, all due to tachinid flies. This has made me do a great deal of thinking.
I did everything right, but they still died. Why? Because that's just the way Nature goes. We might want to interfere with it, maybe help it along a little, but in the end, we can't control it. Believe it or not, farmers and gardeners all around me are doing their best to attract tachinid flies and parasitic wasps to their land, because they help keep other pests down. Ironic, isn't it?
Here's the thing: When we take a tiny slice out of the ecosystem and try to manage it, it gets very complicated, very quickly. One of the biggest problems with looking at Nature from a microcosmic point of view is that we can start to think of the creatures we encounter within it as pets, and so we take it personally when predators attack them, and we can get our thinking twisted, and get emotional about it. But these caterpillars are not pets. Each adult female Monarch lays hundreds of eggs, because the balance of Nature in their world requires a big numbers game to maintain a healthy population. Only a few survive - the lucky, the strong, the healthiest. When we interfere with this balance, we introduce variables into the system, especially if our interference results in releasing butterflies into the population that may be carrying diseases and parasites that never would have made it onto the migration route, or to the over-wintering grounds, had we left the system alone.
I'm now 100% convinced that my efforts will result in more "bang for the buck" if I focus on influencing my community to plant milkweed for the caterpillars, and nectar plants for the adults to feed on. My HOA (of which there are over 25,000 in the state of California) has agreed to plant milkweed on common area land. I've discovered that we have a mandatory committee called "landscaping," which currently has no chair and no members. That's where I'll start.

Comment by Gary Lee on Monday, July 31, 2017
I bought some tropical milkweed from my local Home Depot here in San Diego. I too was concerned about whether they had been treated with any pesticides that might be harmful to caterpillars. From the label on the pots, I was able to determine the plants came from a grower in Vista. I called the grower and one of their staff graciously confirmed that their plants were grown pesticide free. Kudos to Home Depot.

Comment by Kim Henderson on Sunday, August 13, 2017
I became involved in trying to raise monarchs about 2 years ago. I have several kinds of milkweed in various places around the yard. I have had really good luck but this year I have had problems with assassin bugs and tachinid flies. While they may be beneficial insects in most cases, they aren't when you are trying to save monarchs! I have also had a problem with oleander aphids (lots) and fungus on some of my milkweed. I know this isn't addressed in this thread but I thought I would put out some ideas. For the tachinid flies I am going to try planting basil, rosemary and lavender among the milkweed. These plants aren't invasive and help to repel flies. Basil and rosemary are annual in my zone but that is ok if it will help! I have a lot of oleander aphids which I am going to control using alliums, dill, chives, petunia and nasturtiums to help control the aphids. Some of these plants are annual but the diversity will actually help the monarchs by providing plenty of flowering plants around the milkweed. It will make the area really look great and help protect the monarchs. I will still be carefully monitoring the plants and probably bring the ones I find into to the house to make sure nothing gets to them but I am hoping these measures do most of the work for me. I have tried it this year but I didn't have enough diversity and I think I got started too late. Next year will be better! I hope this helps.

Comment by Trudy Bowers on Saturday, August 19, 2017
I live in northeast oklahoma and this is my second year trying to help the monarchs. Last year i planted tropical milkweeds and had great success both outside in the garden, and raising some from small cayerpillar stage in a container with a screen top outside on my balcony. I planted native milkweeds this spring and they were visited in July by several females, who laid lots of eggs. I was ecstatic. But then the beautiful caterpillars began to sicken and die off just at the j stage. I had no idea what was causing it until i came across your web site. I believe i lost every single cayerpillar to these horrible disgusting tachnid flies. I feel so sad, and wonder how i might have unintentionally damaged the local monarch populaion here by creating too much of a monoculture in my backyard. I believe that the comment from Rob Wood, above, is right on. We should focus on getting more milkweed planted in various areas of our communities, instead of trying to turn our backyards into monarch factories. I noticed lots of milkweed on our highway rights of ways this spring, that were later mowed when they were in bloom. That would be one good place to start.

Comment by Jen on Monday, August 21, 2017
I have been raising caterpillars for over 20 years now every summer. We did it as kids every year with my grandma, and now it's something I do with my kids. I have seen what these nasty flies have done to the caterpillars and what they have done while they are in their chrysalis =( we currently have 20...well 19 now. I just experienced something I never have before.....I had a little caterpillar 1/4 inch long walk away from the leaves to do one of his caterpillar shedings....he successfully shed but was not heading back to the plant. I waited the night to see if he was just slow getting back when I realized he was taken over by a maggot. I have NEVER seen one this small produce one. Which brings me to another fact. If the only way they can get this maggot in them is for the fly to do a "fly by" then they can put them in eggs. All of my small caterpillars that I got as EGGS are separate from my big guys until they are about a 1/2 then I put them in the big cage. Well my little guy before he got the chance to move from the little tank to the big tank produced a maggot. They are inside in a closed containers (with venting of course). There is no way that a fly got him after he had hatched from his egg as he was inside my house since before he hatched so that only leads me to believe that they DO in fact infect monarch caterpillars while they are still in their egg. Just thought I would share my story. Oh yeah and I'm trying to get my county to quit cutting the ditches with the milk weed till after September 1st to help get some more caterpillars to the success if living to a butterfly. August is the prime month here in MN for finding caterpillars.

Reply by Steve (Cranial Borborygmus)
Hi Jen, Thanks for everything you've done and are still doing for the Monarch! Kudos to your grandma! If you are saying that you brought several Monarch eggs into your house and then protected them both before and after they hatched into caterpillars, but then one of the resulting small caterpillars died and a tachinid fly maggot emerged, then that would indicate it got injected/infected with the tachinid parasite when it was still an egg. My own results and research have indicated that this doesn't happen, but maybe it does? Hopefully it is very rare though, so that if one can find and collect the Monarch eggs, and then protect the caterpillars after they hatch, the chances of having a tachinid problem are remote. Thanks for sharing your experience, and please let me know if you learn anything else.

Reply by Jen on Tuesday, August 22, 2017
Yes, that is what I am saying. I have a specific tank for just the ones that were eggs and i do not put the caterpillar in the big feeding tank till they are over 1/2 inch long. This guy didn't get to make it to the big feeding tank because he was infected. I had 13 eggs in that tank and all but that 1 made it to the big tank. I have never seen one so tiny get infected with one.

Comment by Margaret on Wednesday, September 06, 2017
After being puzzled by the sudden rise in tachinid deaths among my monarchs, I came across some info regarding common snakeroot:(Ageratina altissima)
"The nectar of the flowers attracts a variety of insects, including leaf-cutting bees (Megachile spp.), wasps, and various flies (Syrphid, TACHINID, & others)..."
Unfortunately for the monarchs in my yard, I recently decided to let this weed grow and bloom among my milkweed, assuming (incorrectly) that it was a benign native plant that provided much needed late-season nectar for adult monarchs. :( But instead of supporting Monarchs, it appears I have unwittingly contributed to their demise by attracting the tachinid flies right to my feeding caterpillars.
While ripping it out may not solve the problem, maybe it will at least lessen the amount of them that I am ATTRACTING in ever greater numbers to my yard...lesson learned. Makes me wonder what other plants they are attracted to. Anybody know?

Comment by Anne Berbling on Sunday, September 10, 2017
I have just this year established native common, swamp and orange (butterfly weed) milkweed in my "butterfly garden" (I didn't realize milkweed seeds needed cold stratification to germinate, until spring - too late), and having no nurseries that carry native plants locally,had to resort to transplanting from areas where the plants would be mown or plowed under. I had just wanted to attract Monarchs to my garden, not actually bring them inside to raise - until learning in a Facebook Monarch group, that that was exactly what people were doing, so I decided I would, as well. I was excited to find my first Monarch eggs just about ten days ago, brought them in, and they hatched! In the meantime, I also found a few tiny caterpillars on the milkweed - along with one larger one - and brought all those inside as well...I had seen mention of the tachinid fly in some of the posts in the group, but honestly thought I was just protecting them from birds... until this evening. We had been gone all day, and the first thing I checked when we got in were the caterpillars (which I, probably unwisely, had begun to think of as "pets"...my little "charges", at least) - and sadly, the largest one ("wild" caught) - was dead. And not just dead, but positively deflated-looking...shrunken. I decided to see if there was evidence as to what killed it, and gently pulled it apart (it looked like a deflated little balloon) - well, there were four disgusting tachinid maggots, which (after showing my husband) I flushed. Now I am questioning the wisdom of this, and wondering if this is going to be the fate of all the rest of the ones (and I haven't kept them separated) that hatched outdoors. So disheartening! If none of these successfully make it to adulthood, I am just going to work on my garden, providing host plants, but not interfering any more with Mother Nature ("red in tooth and claw").

Reply by Steve (Cranial Borborygmus)
Hi Anne, Thanks for your comment. I understand your frustration. I'm confident that the caterpillars you have been protected since they were eggs will be fine. And quite possibly, the small caterpillars that you brought in will be ok too, although they might also have been infected by a Tachinid fly and won't make it. You do not have to worry about the caterpillars "infecting" each other (eg. that the big caterpillar infected the small caterpillars). The only way a caterpillar gets the Tachinid maggots inside of it is if an adult Tachinid fly lands on it and injects a Tachinid egg under the caterpillars skin. I know how easy it is to "bond" with your caterpillars, and then be upset when they die. But it is still also very interesting and satisfying to watch the whole Monarch growth cycle when it is successful. Some people will decide it is best to just grow milkweed and let Mother Nature do her thing, and others will continue to raise Monarch caterpillars inside. There are some interesting comments and thoughts by Rob Wood above on what the best course of action is. I think he had finally decided to just let nature take her course and do other things to help the Monarch, but then the next day emailed me the picture and thought below. Thanks for commenting and sharing your experience and thoughts!

But then, this happens, and the gloom is replaced with joy, if only temporarily.
Had a visitor come to our garden for lunch today. Best regards, Rob

Comment by Joseph Doyle on Friday, September 29, 2017
Hi Steve, I became interested in rescuing the dwindling population of Majestic Monarchs in 2016 when we added some tropical milkweed to our newly redone garden here in Orange County Southern California. I was delighted when I saw Monarch caterpillars. Having a degree in Biology I erroneously believed that they were "immune" from predators since milkweed has toxic cardiac glycosides. I was shocked when we lost them all to our numerous lizards, Towhee birds, and worse to the dreaded tachinid fly. I bought a large butterfly enclosure, stocked it with pesticide free milkweed and began collecting our caterpillars and putting them into "protective custody". Daily I would collect caterpillars, clean the enclosure and remove diseased caterpillars and chrysalides before the tachinid pupae emerge. I have sometimes found reddish-brown tachinid pupae that had hardened and quickly remove and squash them. For the 2016 season we released 40 Majestic Monarch.
We started the 2017 season in early May when the caterpillars began appearing. Because we would plant the eaten milkweed in our garden and they would lushly refoliate we soon had too many cats for one cage. We bought a second large enclosure and began a much larger rescue operation. We formed Monarch Butterfly Rescue and began collecting contributions for the purchase of milkweed and supplies for our rescue efforts. As of today 9/29/17 we have released 79 Majestic Monarchs and have 13 chrysalides and numerous caterpillars of varied instars. Ou goal was 100 rescues and were confident we will exceed it.
One question for you and your readers... on a couple of occasions we have missed finding a tachinid pupa and would find a live fly in our cage, which we would quickly dispatch. Could this single hatched fly infect our caged cats or would it have to mate with another fly first to be egg carrying?? Any thoughts or answers will be appreciated. Thanks, Joseph

Reply by Steve (Cranial Borborygmus)
Hi Joseph, Thank you for commenting and sharing what you are doing to help the Monarch. I remember as a kid learning that milkweed was "toxic" and that other insects, birds, and animals wouldn't eat the Monarch caterpillars because of the milkweed. But I think you are correct that other insects and animals will still eat the caterpillars, at least initially. They might learn after eating one or two that the caterpillars don't taste good, but in the meantime, probably at least some caterpillars do get eaten as part of the "learning experience". I have a lot of lizards in my yard. I've never seen a lizard eat a caterpillar, but I've wondered...
As for your question, I do believe there are male and female tachinid flies, and they need to mate before the female can lay eggs. So if there is only one fly and you get rid of it, you should be ok. Thanks for all your efforts for the Monarchs!

Comment by Lisa Buick on Monday, October 02, 2017
Have had several caterpillars make it to butterflies. But this Fall, many of them were infected with the horrible Tachinid fly larva. We have several milkweed plants in a small garden along with butterfly bushes which are huge. I'm wondering if the fly problem is more prevalent because the butterfly bushes are so close. First year (3 years ago), we had tremendous success. Now that the plants have grown, the tachinid fly problem is terrible. Part of the problem also is that I can't recognize if a caterpillar has been infected. We bring them into a screened enclosure on our screened porch, but they must have been infected already (we brought them in when they were caterpillars. Tried to do the eggs with no success.
Also, you mention you bring fresh milkweed leaves in each day. I did not do that. I cut a stalk and brought that in and now I believe that was a big mistake. I thought I read that those flies don't lay eggs on the milkweed plants, but they must have some how done that. Anyway, I think I have a lot more to learn (obviously). Just wondered if you wash the milkweed leaves before you bring them in for the caterpillars to eat.
Thank you very much for posting all the comments and answers here - they have been very helpful.

Reply by Steve (Cranial Borborygmus)
Hi Lisa, Thanks for your comment. I still believe the Tachinid flies only lay their eggs on/in the caterpillars and NOT on the Monarch eggs, but they can lay eggs on/in very small caterpillars. One or two readers have disagreed with me though and believe that the Tachinid fly can lay eggs on/in a monarch egg. Again, I don't think this is the case. If you can find Monarch eggs and then raise the caterpillars protected from exposure to any flies, they should be ok. I raised my caterpillars in small sealed containers (with mesh screen lids for air flow). There was no way for a fly to get to the caterpillars. I fed each caterpillar a leaf or two at a time because I didn't have a lot of milkweed at the time, and didn't want to waste any leaves. The leaves also went limp within a day. I never washed any of the milkweed. If you want to use whole stalks with leaves, that should be ok. I don't think you need to wash it. The important thing is that you have to keep any flies completely away at all times. You are also correct that if you find already-hatched small caterpillars, there is no way to know if they've already been injected with tachinid eggs or not. Good luck!

Comment by Larney Grenfell on Wednesday, October 25, 2017
Hi Steve, I live on the South East Coast, Australia and have been raising caterpillars off and on for years. It is only this last lot over the last 5 years or so that I have noticed flies emerging from chrysalises. They seem to attack the Orchard Butterfly here Papilio aegeus aegeus Donovan or Orchard Swallowtail and the Papilio fuscus. I have raised quite a few other species but have not noticed the problem though maybe that is just coincidence. We did have quite a few Wanderers or Monarch butterflies here a few years back but not so many now. Not sure if this is because of flies or not. Have not ever needed to keep them inside. I have found if I get the eggs of butterflies I don't have a problem but if I collect the caterpillars then it is touch and go. Some of the littlies survive but if they are bigger then usually flies come out. I am not sure if it is the same fly we have here but I have maggots in waiting at present from two caterpillars just now and will soon know. I would have called the flies I saw come out of mine last year 'blow flies' as they looked bigger and not so hairy as the photos you have but will be more scientific this season and identify them. The reason I started keeping caterpillars was because I read that only 4% of eggs laid ever make it to full adult stage because of birds and the like so I thought I would give them a leg up seeing as we have changed the way we encourage birds to our gardens.
I tried to get the fly traps you indicated from Amazon but they won't send them to Australia so will make inquiries and try and get something the same here.
Is this all because of the imbalance we are causing using pesticides, the fact more species are introduced through escaping quarantine or just that weather is changing and the warmer weather at different times is unsettling our insect populations. Wish I knew but things are definitely changing. I used to see fire flies once but never see them here these days. What else are we destroying with out practices of humanising everything. I know we have a lot of trouble here with our Richmond Birdwing butterflies because people planted the wrong vines The Dutchman's Pipe, in gardens which they laid eggs on and then it poisoned them. There is a big push to plant the right vine now and it is having some success.
Glad I found your site would love to keep up to date with what people are doing. Cheers

Reply by Steve (Cranial Borborygmus)
Hi Larney, Thanks for your comment and report from Australia. It's great to hear what's happening with the butterflies "down under"!

Comment by Emily on Thursday, October 26, 2017
I live in Santa Clarita, CA. This is my first years raising Monarchs. I LOOOOOVE caterpillars. I have had great success with my my cats. I have lost a few of my babies to this fly. It is heartbreaking and it takes me a while to get over it. On the other hand, I am ever so careful to try and collect the eggs as I follow the Monarchs through my garden. I have also lost a couple of Butterflies to OE. Again, I am VERY careful to clean up and disinfect anything a butterfly touches after I release it/them. I thought about testing my butterflies for OE but, I do not feel that experienced yet.
It is difficult to not collect the teeny baby caterpillars too. I believe this is where I am picking up the few infested cats. So far, I have released about 65 butterflies. I am currently raising about 100 cats. It is a lot of work but so worth it! I make sure to thoroughly wash all of my milkweed. I also make sure to clean out my indoor cages sometimes up to twice a day.
And lastly, I also found Giant Swallowtail caterpillars on my rue! I am so crazy about the "Bird Poop" caterpillar. It is fascinating! Wish me luck with my 6 cats & 2 chrysalides.

Reply by Steve (Cranial Borborygmus)
Hi Emily, It is a lot of fun. I've only ever seen a few Swallowtail butterflies, and just one Swallowtail caterpillar I think. They are also very interesting. Good luck with your cats!

Comment by Sandy on Saturday, February 10, 2018
Thanks so much for this blog and all this information. I live in Baltimore Maryland and have not experienced this fly. I did bring in 2 eggs last year to raise inside. I wanted to comment that all 3 of the eggs that I brought in last year hatched, lived a day or so and then died. They looked like they dried up. So I did some research and realized that I should have kept the leaves moist somehow. Maybe I should have set them on wet paper towels or put the stems in water. It was heartbreaking so I am passing on this info to others who want to raise eggs inside. In the previous year I brought in about 15 caterpillars which successfully became butterflies. They ranged very small in size up to big fat ones.
Steve, here is a link to my blog post on this experience.

Reply by Steve (Cranial Borborygmus)
Hi Sandy, Thanks for your comment and good advice.  I enjoyed your blog post. Keep up the good work!

Comment by Marcia on Saturday, February 24, 2018
I live in Kailua, on the island of Oahu in Hawaii. This is my fourth year of raising monarchs and interestingly, it has been my most successful year in terms of the number of releases (probably 300 since December). However, this is the first year I've had issues with the Tachnid fly.
Typically I bring in caterpillars when they're at least 1/4" or so, but after reading all the comments here, maybe I need to gather the eggs as soon as possible. I'm going to check for more right now! This site is so informative as now I understand more about this nasty pest. Keep up the good work! Mahalo!

Reply by Steve (Cranial Borborygmus)
Hi Marcia, Thanks for commenting! It's great to hear about Monarch caterpillars and butterflies in Hawaii, although it's sad to hear that the Tachinid fly is there too.

Comment by leslie williamson on Saturday, June 23, 2018
Luckily, in the past 2 months we've had 51 beautiful, healthy monarchs fly free from our garden. 2 more still pupating, and now a new generation starting up. It's been somewhat cool so far, and not a lot of casualties. As it heats up I fear that the nasty tachinid fly will become more active, and destroy our babies. Wish us well w/our precious, beautiful creatures. Thank you

Comment by Patryk Battle on Wednesday, July 25, 2018
I love that so many people are growing milkweed. And hope that many of them have noticed that it is not only Monarch butterflies that benefit from this amazing plant. A whole array of soft bodied insects make this plant their home including the milkweed aphid and the milkweed seed bug which is soft-bodied as a nymphs later on as adults not all . The result of this wonderful diversity is a great array of beneficial insects: predators and parasitoids included. Although I may be pilloried and despised for saying this absolutely the Tachinid fly is a beneficial insect. Makes me sad people are doing so much to enhance diversity but are still caught up with a God complex. The world works if we allow natural diversity to thrive. The Monarch will thrive as it always has despite the presence of Tachinid flies. The problems are neonicotinoids, habitat destruction, and global climate disruption it is not Tachinid flies. I have also been allowing milkweed to thrive. Sadly this is the first year in five or six years that I'm seeing monarchs in my patch. I will not be policing for Tachinid flies or vilifying them but next year I will planting many other sources of nectar and pollen and indeed water such as: Sylphium perfoliatum, Tihtonia, buckwheat, cowpeas etc. Please folks protect and revel in the glory of the garden that is this planet and understand that we do not know nearly enough to pick and choose its inhabitants.

Comment by Adrienne McKinney on Friday, July 27, 2018
I have loved Monarchs my whole life. As a kid growing up in Hawaii we would bring them in and watch them hatch. Hawaii has amazing milkweed trees. Calotropis Gigantea. In Hawaii we call it purple wax crown flower. Now that I have kids of my own I have started up an old hobby. My husband calls me the butterfly lady or hoarder depending on how many pillars I have on our table. I garden at home as well as run the elementary school garden here in San Diego. Anyway long story short I've been maintaining the tropical milkweed bushes that were already in the garden for about two years now. We have the pesky flies too. What I have deduced from observation, is the flies get worse as it gets hotter. My spring batch of pillars all did well, almost no issues. I had butterflies being released all over campus everyday till school let out and very minimal loss. The kids and teachers love it!
But now that it's almost August I have had so much death. I bought a bunch of milkweed to plant over the summer at school, but right after I bought it we had our first heat wave and with school out I can't water everyday so I kept the bushes at home. Well lets say it isn't going well. I brought in all the pillars the other day because not but a single one has made it. Since July something when I got the bushes. I brought 36 pillers in and in three days I'm down to 12 or so. I'm bringing in the leaves every couple of hours or so but I also have the yellow aphids. Someone said that they didn't think the aphids bothered the pillers much. I rinse them off anyway. But the other day we observed one lone aphid I missed and sure enough it was running up and down the stem of the leaf bitting the pillars. Do you know if the aphids are an issue too? I have some of my pillars just dying for no reason. The other day I had this great idea...lady bugs kill aphids naturally...right? Well they also kill pillers too, so back to the drawing board.
Anyway found your blog trying to find a solution to the problem. Based on the comments there may not be an answer. It's just super disheartening for me and the kids when the pillars don't make it. I mean if you're going to take the wings from a butterfly the least you could do is be magnificent. Stupid flies.

Comment by Craig - Southern California on Friday, July 27, 2018
Wow! Your web page had such great information about the Tachinid fly. I have a few milkweeds in medium pots, and they are full of Monarch caterpillars right now. I watched in shock as I witnessed some Tachinid flies injecting my caterpillars and they would fall off the milkweed leaves writhing in pain. This happed so fast I could scarcely believe my eyes. If not for your web page I never would have had any idea about what it is that I had witnessed. I purchased some "Rescue reusable fly-traps" which I found at my local Home Depot, and you can also find on-line at Amazon. I looked up what attracts these dreaded flies and am trying to figure out what attracts them the most. I took some cilantro, and chopped it up, put it in the Rescue fly-trap and filled it halfway up with water. I took another Rescue fly-trap, put some shredded carrots in the bottom, and filled it halfway up with water. I have these very small garden hooks, that I used to hang these fly-traps just above my milkweed. In theory, if the flies are attracted enough to these scents they will fly into the container, they will be unable to fly out and they will drown. There are other scents that I will be trying in the future, dill weed being another one. I may also try to boil these plants into a more concentrated form. These reusable fly-traps come with a water soluble non-toxic bait that really lures regular flies, but I don't think that this will work for the Tachinid fly, but I am open to be trying it in the future as well. If a Monarch lover could develop a Tachnid fly pheromone, that may prove to be a major lure. If a major Tachinid fly attractant can be found, and put it in some of these reusable fly lures, maybe we can keep this fly population down very low or down to none at all. At least around the Milkweed plants. I also saw someone's Tachinid fly lure formula as: 4 cups water, 2 cups sugar, 1/4 cup honey, 3 tbls Boraxo, so that might be worth trying also. For the larger Milkweeds you can use full-size garden hooks, found at any nursery. I also saw a larger "Rescue pop-up reusable fly trap," and a "Victor reusable fly trap" that all work on the same principal.
Thank-you for your informative web page, and I will keep you informed on my progress. If anyone else comes up with an irresistible Tachinid fly lure, please share it here.

Comment by Craig - Southern California on Sunday, July 29, 2018
I decided to use a blender, and blended some fresh cilantro, fresh carrots, dried dill weed, dried coriander, I then placed this smoothie in my "Rescue fly traps." I also made a couple more fly smoothies and added some sugar and honey, just to see if these might attract the Tachinid flies. Maybe as this ferments in a few days, the flies might think it's a bar and stop over for a drink. (Please come in and drown?) I have been observing my few small milkweed plants and it looks like these flies move in a very fast zig zag motion, hit their caterpillar target and then do the same thing over and over. They are relentless and don't rest much. They also seem to be iridescent gold or blue/green that show up well in the sunlight, but that may be common to all flies. I stood by with an old fashioned fly swatter, and kept moving it towards the flies in hopes that they may get tired and take a rest in a spot where I could swat them. I was actually successful at this a few times. I used a magnifying glass to identify my kill. In the meantime I will keep trying to find an attractant, that the Tachinid fly can't resist. I can tell you that the smelly regular fly attractant formula that comes with the "Rescue disposable fly traps" attracts regular flies so well, the traps will actually become full of drowned flies in a very short time. My ultimate goal!
One more thought: If you raise Monarch caterpillars and they die because the Tachinid fly got to them, and you freeze these caterpillars, or make sure that all the Tachinid larva are destroyed, you will have prevented about six more flies being released into the world each time. You can take a little comfort that the Monarch caterpillar didn't completely die in vain, and you have helped decrease the Tachinid fly population in a small but significant way. I call this: The Tachinid Fly Effect.

Comment by Craig - Southern California on Wednesday, August 1, 2018
I checked my "Rescue reusable fly traps," and I don't see a single fly in any of the three that I placed amongst my milkweeds. I purchased two "Rescue disposable fly traps," and in less than twenty-four hours, I am beginning to see results. All of the "Rescue fly traps" use the same attractant, so I can only assume that the reusable fly trap, with four small holes on the top of the green lid is a poor design. If a Monarch enthusiast was to place some of the disposable fly traps on garden hooks around their milkweeds, and when the trap has a lot of dead flies in them, cut them open (yeah gross) and identify if the trap has killed any Tachinid flies, and do a number count, that would be very useful data indeed. You have to be really dedicated to take on this challenge. I have read that the non-toxic fly trap attractant smell is really terrible, but it doesn't seem that strong to me, and smells a little like moldy old tires. Maybe this is a new formula. "Rescue" has a reusable pop-up fly trap with the same type of lid as their disposable fly traps. This looks like a better design. I will purchase some, and test them with what I hope may be Tachinid fly attracting smells, like concentrated fresh dill, cilantro, parsley, queen anne's lace, lemon balm, liquefied in a blender with water added, and placed in the reusable fly traps. To date, I have lost at least fifteen captured Monarch caterpillars to the Tachinid fly, and I would say there were at least probably another thirty that I didn't capture, and were also victims of the flies as well. I know that I had at least five Monarchs that made it to full maturity, and left to fulfill their destinies.

Comment by Rob on Saturday, August 25, 2018
I've been doing a lot of research and running experiments in an attempt to gather enough data to address the Tachinid issue. I bought a fairly powerful digital macro microscope that's connected to a laptop, and this has helped shed light on these creatures...

Tachinid Facts
The University of Minnesota ran a 17-year study in conjunction with hundreds of MLMP (Monarch Larva Monitoring Project) citizen scientist volunteers across North America. These volunteers collected Tachinid larvae that emerged from parasitized Monarch caterpillars they were attempting to raise. They isolated each dead caterpillar's group of larvae in separate containers, allowed the larvae to pupate and hatch as adults, then froze the flies to euthanize them. Each group was labeled, a full description written of when and how the samples were collected, and then sent to the university's Department of Fisheries, Wildlife and Conservation Biology for identification and cataloging. In all, >20,000 samples were sent to the lab.
Results High Points
7 different species of Tachinids were identified as having parasitized the caterpillars in the study.
In some samples, multiple species of Tachinids had parasitized the same caterpillar.
5 of the 7 species either lay eggs on or (by injection) within the caterpillars, or deposit hatched first instar larvae onto the caterpillar. The overwhelming majority - but not all - of the caterpillars in the study were parasitized in this fashion.
2 of the 7 species (H. virilis and Leschenaultia sp.) lay nearly invisible, microscopic eggs on the milkweed, which are then either ingested by feeding caterpillars, or hatch into larvae which wait patiently for a caterpillar to pass by, latch onto it, and immediately burrow into it. No direct parasitization of Monarch eggs was observed, but "absence of evidence" is not "evidence of absence." Without a powerful microscope, persistence and luck, a Monarch enthusiast is not going to discover these microscopic eggs, regardless of where they're deposited. The ramifications of this last piece of data are important, because even if we collect eggs, the leaves or leaf segments we collect with them, or which we later introduce as feed, may have H. virilis or Leschenaultia sp. eggs on them. Unless we thoroughly clean the leaves in either case, we may be accidentally introducing Tachinid parasitization into a habitat that we have assumed would protect our caterpillars from it.
My Experiment
I collected 70 eggs over a 24 hour period, and brought them into a spare bedroom, in a closed container. This container was never exposed to any type of fly. Many of the caterpillars that hatched from these eggs eventually died in early stages of development (first and second instars), with no obvious symptoms: They ate, grew, and died, appearing to be healthy, normal caterpillars one day, and lying on their sides, dead, the next day. The surviving caterpillars continued to grow, and then a 3rd instar died in the same way as the earlier casualties. I decided to carefully dissect it under the microscope, and discovered 5 tiny Tachinid larvae inside it, each smaller than the head of an ordinary straight pin. I subsequently discovered Tachinid larvae inside a second caterpillar that died as a fourth instar. It may be that many of the early deaths were also caused by Tachinid parasitization, but I didn't at that time have the microscope needed to see them. I saved all of the larvae from these two samples, which later pupated and hatched in separate containers. The species have not yet been identified by the lab.
On Trapping
I'm skeptical that trapping is an effective means of controlling Tachinid flies. In order to trap them, it is first necessary to attract them. When we deliberately set out to attract the very insects we're trying to control, we may simply be adding to their numbers in our garden. Yes, we may possibly end up with some Tachinid flies in these traps, but more than likely, many of the flies will be houseflies or other species as well. I have some friends who are experimenting with various concoctions of liquid attractants, but I don't hold out much hope for this solution. I'm keeping an open mind, though.
Of all of the various ideas I've discussed or thought about, this is the one solution that I think might be the most promising. If we can find a substance or even a mechanical device (sonic?) that repels the Tachinid without harming any creatures at all, then that would be the perfect solution, wouldn't it? The Tachinid flies would simply not be interested in visiting our gardens, and we could raise Monarchs in peace. At any rate, that's my next line of inquiry.
Weather and Temperature
This year, my garden was blessedly free of Tachinid flies until July, when temperatures began to soar here in San Diego County. Suddenly, aphids appeared, and so did Tachinid flies. I've read all of the comments here, and virtually all of them are dated from July onward. What if the ambient temperature has to exceed a certain number (90 degrees F?), before the Tachinids can reproduce in numbers sufficient to grow out of control? Could there also be a direct corollary with the appearance of aphids? I know that Tachinids love the honeydew that Oleander aphids excrete. In theory, if we keep the aphids down, we could test this. I've been working on this in my own garden, very religiously, so we'll see.

Reply by Steve (Cranial Borborygmus)
Hi Rob, Wow, thanks for your comment. The University of Minnesota research study results (and your own experiment results) are very interesting! Thanks for sharing. I learned a lot from your comment. Thank you!

Comment by Rob Wood on Friday, August 31, 2018
Tachinid Experiment
Hi Steve, just thought I'd let you know that I've just collected several dozen Tachinid fly pupae from a fellow Monarch rancher, and another 2 dozen from my own parasitized caterpillars. My plan is to allow these pupae to hatch in a controlled environment, and feed them to keep them alive through their natural lifespans. The experiment has two objectives: 1) find a substance that attracts the flies, suggesting further experimentation for development of an efficient bait for traps, and 2) find a substance that repels the flies - but not butterflies - that might lead to a practical garden accessory to keep Tachinid flies away from milkweed plants. Wish me luck!
Rescue disposable fly traps
Several Monarch ranchers here in North San Diego County have tried the Rescue disposable fly traps in their Monarch butterfly gardens. The used one I looked at was full of green bottle flies and common houseflies, and smelled like rotting meat and decaying vegetable matter. The manufacturer states, "The RESCUE! Non-Toxic Disposable Fly Trap catches common nuisance or filth flies, hundreds of the most prevalent species including house flies, false stable flies, blow flies, blue and green bottle flies, flesh flies, face flies and many others." These flies all feed on filth and decaying organic materials. Tachinid flies, on the other hand, feed on fresh nectar and Oleander aphid honeydew. I don't see how this trap could work to control Tachinid flies.

Reply by Steve (Cranial Borborygmus)
Hi Rob, Thanks for your follow-up. Your experiment sounds very interesting (and like a fair amount of work - you are dedicated to this cause!). It would be great if you can determine either a better attractant/bait for using fly traps, or a plant that would discourage the tachinid flies from hanging around. Good luck. I'll be very interested to hear what you learn!

Comment by Ulrike on Wednesday, September 05, 2018
Tachinid egg removal
I recently found a 5th instar wild caterpillar outside on my milkweed that had seven tiny whitish-yellowish dots stuck on it, perhaps the size of half a poppy seed. They looked still plump, so as if maggots were still inside. I surmised that they were tachinid fly eggs (poor caterpillar), so I brought the doomed cat inside to see if these could be removed somehow. I indeed managed to scrape all visible eggs off with a very small penknife and tiny tweezers - without hurting the caterpillar. Looking at the tiny eggs, most looked like they were still full, but one looked empty. I used a magnifying glass to see if there were any others, then put the cat inside a jar with some milkweed. Despite being stressed, it ate some, the next day it hung itself in a j-shape and after about 16 hours it became a chrysalis. Unfortunately, two of the maggots must have indeed left the egg and entered the caterpillar before I removed it, since two tachinid maggots emerged from the chrysalis a few days later. :( But I believe that the flies lay the eggs on the caterpillar, not inject it. Removal of the tachinid egg is theoretically possible and survivable by the caterpillar, though likely will stress out the caterpillar more than necessary. .

Reply by Steve (Cranial Borborygmus)
Hi Ulrike, Wow. Thanks for sharing your experience. Very interesting!

Comment by Suzanne Bennett on Monday, September 10, 2018
You might try more diversity in your garden to distract Tachinid flies from your monarch caterpillars. If all you have is lots of milkweed and lots of monarch caterpillars, it's sort of like a monocrop. These flies parasitize all manner of caterpillars and other critters. Try growing some tomatoes and squash and cabbage so that you will also have tomato loopers, squash bugs and cabbage loopers on offer (just to name a few). It may divert them!

Comment by Rob Wood on Monday, October 08, 2018
Update on tachinid fly experiment results in North San Diego County: Some Monarch caterpillars become parasitized, even if collected as eggs and subsequently raised in protected enclosures.
At least two tachinid species (H. virilis and Leschenaultia sp.) in North America deposit their eggs on milkweed. As Monarch caterpillars eat the leaf, they accidentally ingest the eggs, which immediately hatch inside the caterpillar. These eggs are microscopic in size, and are not visible to the naked eye.
From the report published in July of 2017 by the University of Minnesota following the 17-year joint university and MLMP tachinid/Monarch study: “Because there is no evidence that any tachinid flies attack host eggs, this finding indicates that milkweed foliage 'infected' with the microtype eggs of H. virilis was introduced by MLMP volunteers, either on the leaf on which the egg was collected or subsequent leaves collected for feeding.”
My conclusion: The implications of this finding has far-reaching impact on Monarch rearing, at least for those who collect milkweed and Monarch eggs or larva from outdoor settings. It means that at least in some cases, we are unknowingly introducing tachinid larvae into our protected Monarch enclosures. The question is: How can we prevent the introduction of these eggs into our habitats?

Reply by Steve (Cranial Borborygmus)
Hi Rob, Thanks for your continued research and reports!

Comment by Craig (Southern California) on Saturday, October 13, 2018
I did a little internet research and found a few, very hard-for-me-to-read agricultural/scientific, research papers that seemed to suggest that Tachinid flies were attracted to (methyl (E,E,Z)-2,4,6-decatrienoate) a stink bug attractant. These documents were difficult for me to read and I just thought that I would at least try to use the information as I understood it. I purchased the latest stink bug attractant from Rescue, that has two pheromone packs in each container packet, each pack is supposed contain different stink bug pheromones. I placed double stick tape inside my Rescue reusable pop-up traps, so that the attractants would be near the opening. These pheromones are supposed to be good for about 90 days. I don't know if the Rescue stink bug pheromones contain (methyl (E,E,Z)-2,4,6-decatrienoate), since Rescue doesn't list any ingredients. After watching the two traps in different milkweed locations, I can say that I have not seen one single Tachinid fly trapped. I did get a Milkweed bug who fell in by accident, and I released it. I purchased the Rescue stink bug attractants through Amazon, and a large amount of reader feedbacks state that the attractant didn't even work on stink bugs. This was the only commercial stink bug attractant available. I also contacted a professor, who is an entomologist in Davis California, and specializes in Monarchs and milkweeds. He listened to what I had to say, and suggested that (as stated in an earlier comment), if you were able to find a powerful Tachinid attractant, you may capture some flies in your traps, but the "extra flies" which would now be attracted to your milkweeds may actually make your Tachinid fly invasion worse, and result in the deaths of even more Monarch caterpillars than before you placed the attractant.
Summary: July 2018 Southern California - "It was the best of times, it was the worst of times for Monarch caterpillars."

Comment by Joan on Thursday, June 06, 2019
Hi, I found this thread while looking up this fly. Here in Tucson my Milkweed (five plants, two species) is home/food to honey bees, Paper wasps, tarantula hawks, flower flies, ants Milkweed bugs, Queen butterflies, all of which with the bug exception acting as pollinators. I've only seen two Monarchs who seemed to be passing through for food. I was looking up the flies, because I am a photographer of bees and pollinators, and I've noticed these flies (among other flies) acting as Pollinators, especially in love with Woolly Butterfly Bee Bush, Cholla, Blue Mistflower and Silver Spurge. I wish I had caterpillars to raise. As it is my garden is filled with insects, rodents, birds, reptiles, all in balance with my plants. All blessings to all of you.

Reply by Steve (Cranial Borborygmus)
Hi Joan, Thanks for your report on milkweed in your area of Tucson. Your garden sounds incredible, and your observations are fantastic. Thanks for commenting!

Comment by Kris on Sunday, June 30, 2019
Thank you for this website, Steve. Very informative, great photos. I too found this website while researching the tachinid fly. I have had success raising the egg to caterpillar to chrysalis. But, my husband found two 5th instar caterpillars on the milkweed at the park where he walks our dog. He brought them home and I put them in an enclosure with milkweed. They didn't eat much, then did the j-hang -- and both had the fly maggots. Bummer, but I wasn't too surprised. The flies here in Southern California are quite prevalent and this is why I decided to raise them from eggs the past two years. The best chance the caterpillars have is in the early spring, before the flies arrive. In nature, the milkweed is eaten and the resulting butterflies migrate to a new area ahead of the flies.
A few suggestions:
Grow your native milkweed for your area; consider it an annual for the season, and hope that the butterflies stay ahead of the flies.
Or, if you want to raise the caterpillars, purchase a container milkweed plant from a trusted nursery (trusted = no pesticides). Set outside for a few days for the butterflies to lay eggs on, then put the entire plant inside a mesh enclosure. My husband built one for me that is 10" x 15" x 25" tall, large enough to fit a 3-gallon size container plant inside. (Learn from our experience: use the fine mesh. The first mesh we used, the flies were able to squeeze through.) He made a removable plywood top for them to hang their chrysalis on. The container doesn't have a bottom on it; it is fairly lightweight and I set it on a cafeteria-type tray that is easily cleaned. Mine has a door, so I can water and mist the plant. The butterflies leave by the door, or I can hang the removable top on the porch.
I do like Suzanne's comment about growing tomatoes, squash and cabbage for the flies to have other caterpillars to infect. In fact, on one website, the fly is listed as a beneficial insect, lol. Just like ladybugs, people can purchase them and release in the garden. Yikes.
And I agree with Craig's entomologist/professor that it is best to not have a pheromone that attracts them, even to trap them.
My next project will be to make homemade fly paper and put inside an enclosure with mesh large enough for the flies but too small for bees and butterflies.
Again, thank you to Steve and also to all those who comment. We learn from each other!

Reply by Steve (Cranial Borborygmus)
Hi Kris, Thanks for your great comment and suggestions!

Comment by Tracy (San Diego area) on Friday, July 05, 2019
The photos here are very appreciated! I have a fairly simple and highly effective way of combating these flies and other predators, such as praying mantis. Last year I raised 144 monarchs this way: All of my milkweed is in pots--about 20. As soon as eggs are laid, I bring the pot indoors, in a bright room with an open window. The caterpillars are raised here, usually in groups of 6-8 pots. When they wander, it's usually to an adjacent pot. I have "onramps" of wood trim that go from the pots to a nearby fake tree. The cats climb up here when they are ready, and I simply move the tree outside when I see that the butterflies (usually a group of 3-5) are ready to come out. Even if I miss one, monarchs dry out their wings for several hours before flying so the tree can still be moved. Once in a while a rebel will choose another location, like the windowsill, and that chrysalis can be carefully pulled off (with silk attached) and added to the tree with masking tape. If I need to regrow the plant, I'll keep it inside so that no new eggs can be laid on it for a while; otherwise, it goes out again to get the next generation.

Comment by Dan S. on Thursday, July 18, 2019
I planted several types of Milkweed in my yard. This year is the first year I have seen the Tachinid flies.
What I do is look for the eggs on the leaves every day. I then remove the leaves with eggs and put them in a small container with just a small amount of water in the bottom to keep the leaf moist. When the caterpillar emerges I put it in an screened container with fresh milkweed. Be sure to remove anything that does not look like a leaf. I put fresh milkweed in the container that I made from Tupperware. I put fresh water in it and punch holes in the lid to fit a stem with leaves. You will need to replace it almost everyday with new milkweed as the caterpillars grow. I keep them until the butterfly emerges from the chrysalis. After I release the Monarch I watch to see if it lays new eggs on my milkweed and the process starts over.
You can purchase a nice enclosure for raising caterpillars to butterflies on Amazon. Good luck!

Comment by Mark. on Thursday, July 18, 2019
Very helpful blog. I live in Massachusetts and am starting to see the parasitoid in my milkweed patch. I see they're attracted to the same milkweed blooms that monarchs like. So I'm going to try cutting all the milkweed buds OFF (too late for this year, but next year, I won't let them bloom at all). I've planted other nectar flowers for the monarchs, so the butterflies (and even the flies) can go there to drink nectar. Since the monarchs will find the milkweed to lay their eggs, keeping the nectar sources in a separate place may help more of the cats survive. The flies now won't find nectar on the milkweeds where the cats are.

Comment by Donna on Wednesday, August 07, 2019
Found this site searching tachinid. Very informative, thank you. I have been raising monarchs the past few years and have seen an uptick this year in infestation from this fly. I usually only bring in eggs and 1st instars, but have brought in other instars this year, keeping them separate from the ones raised from eggs. Almost all the 4th/5th instars I have brought in have been infested. I found a J hanger today on my swamp milkweed with strings hanging from him. There were 6 other large cats on the this plant so I brought them in, hoping they have not yet been hit. We'll see. I have noticed as someone said earlier, the aphids have taken off in this upper 80/90 degree weather. I hosed all them off or cut back the tops of the milkweeds there were loaded with them. I have also had several cats this year that seemed to be doing fine one day and then shrinking up dead the next. I released 370 last year with next to no casualties of cats or chryalides. This year is turning out to have its issues.

Comment by Maggie Sue on Tuesday, August 13, 2019
I've been hit with the dreaded Tachinid fly this year as well. I had 11 chrysalis successfully hatch and then, BOOM, 5 caterpillars died; never able to make the J shape. I found several maggots and brown "beans" and the tell tale strings. I currently have 21 chrysalis which all appear to be okay and another 15 caterpillars 4th-5th instars. I have several newly hatched instars but 2 of them are not interested in eating at all. I'm curious if they're infected. Has anyone experienced instars not interested in Milkweed? I live in northern NJ.

Comment by Lamet Keller on Friday, August 23, 2019
Thankfully I stumbled on this page. So informative. I always thought it was some type of wasp killing my monarch. I plant about 100 plants every year and used to have tons of caterpillars and chrysalis and then it seemed like I had just made a smorgasbord for something to kill them. Glad to finally find out what they are. They tachinids look like what we call blow flies, are they the same thing? And just curious on the fly traps, put them by the milkweed? And does removing the flower help? I don't know that I have seen flies on the milkweed. This is a great site thanks again for all the info. Oh and I'm not sure what an ,,,instar,,, is I saw that on several comments.

Reply by Steve (Cranial Borborygmus)
Hi Lamet, Thanks for your comment, and for all the milkweed you plant! Regarding your question, there are several different types of tachinid flies. The ones we have here in California are shown in the picture I took at the top of this post... a dark brown or black fly with maroon-red eyes. They probably also look like what you are calling blow flies. But we also have common house flies around here, which are different from the tachinid. The tachinids are little smaller than a house fly, in my experience. If you find dead caterpillars or chrysalises with the telltale "white strings" on them, you have tachinid flies in your area. As for the term "instar", it refers to the stages a caterpillar goes through. They grow for a while, and then shed their outer skin, and then grow some more. Each time they shed their outer skin, that starts a new instar. The monarch caterpillar goes through seven instars (I think) before it pupates into a chrysalis.

Comment by Carrie on Monday, September 09, 2019
We live in Branson West, Missouri and have had a horrible time with tachnid flies this year. Last year my mom was able to raise & release 100 butterflies in the fall. This year, we have had only around 30 or so make it to chrysalis stage and we've lost 1 chrysalis. We wondered if the flies were worse this year because of the milder winter we had last year. Would love to find a way to keep the flies away from the monarchs!

Comment by Patti Farris on Monday, September 16, 2019
I was very happy to see this post in a way; lost five chrysalids to tachinid flies, but was told they must have been infected in my house because they only attack larger instar cats. I was sure that wasn't true and you had the same experience so that is reassuring, sort of. Got the cats from outdoors, but they were only about third instar so still fairly small. Thanks, Patti

Comment by Sandra O'Dell on Thursday, October 03, 2019
Great article - It was very helpful. I live in Southern California and this is my second year raising monarchs. I've encountered the Tachinid fly from bringing in smaller cats into my meshed cages inside my home, and also from viewing the milkweed outside. I decided this year to only raise the eggs inside my home. If I see a caterpillar outside in the milkweed, I leave it there. I started late this year, in July, but have successfully raised 80 monarchs to date. Last month, I found numerous eggs on milkweed seedlings that I had planted, and then I enclosed the planter in a extra large mesh cage and left it outside. Once the eggs hatched, I brought the caterpillars into the house. I was hoping to raise them outside in a mesh cage but I've seen too many flies. From my experience, I believe that the flies can "inject" the caterpillar while it climbs on the inside of the mesh cage if the cage is outside. And if a fly enters ones home, it can do the same. Now, I have a total of 41 caterpillars in one very large mesh cage. It will be interesting to see if any die from the Tachinid fly. Since they were enclosed in the mesh cage and all on the seedling plants I believe a fly would not have been able to reach them. From reading this article, I am going to purchase and use some fly traps. Its so disheartening to see flies on my outside milkweed plants. Thank you for your article!

Comment by Patti R on Tuesday, October 15, 2019
Last year I did not have the problem with Tfly but this year totally different story! I rear my Monarchs inside from eggs but when I find a caterpillar I brought it inside also. This year I had to stop bringing in caterpillars. 50% of them were infected with Tfly. In beginning of August I decided if I brought in caterpillars I would quarantine them from others. I had read that when they are 4th-5th instar is when they get infected. Nope I had a couple as early as 3rd instar. So just wanted to throw that out there.
I found this year that bringing only eggs inside to rear was the answer. I released 588 Monarchs and would have had at least 100 more if not for the dreaded Tfly. I hope we can find a healthy way to repel these flies. Until next season!

Comment by Marcia Morales on Saturday, May 30, 2020
This info and the photos are extremely helpful. I am in San Diego. I have had success with raising caterpillars in the mesh cages. I try to bring them to the cage as soon as possible. But the T-fly has gotten many of the outside ones, and even some inside ones (probably gathered them too late). I remove and freeze them once I see signs of failing.
I have a question - once the chrysalis has hardened - can the T-fly still infect it? I have chrysalises on the pots, and would like to take them out of the cage so I can add fresh plants. I apologize if this question has been addressed already. Scanned the comments and didn't see anything.

Reply by Steve (Cranial Borborygmus)
Hi Marcia, As far as I know, once the chrysalis is hardened, tachinid flies do not and can not inject eggs into it (like they do to a caterpillar). However, if the caterpillar was already "infected" with tachinid eggs, depending on timing, the tachinid larva can emerge either while the caterpillar is still a caterpillar or after it has become a chrysalis. In either case, one will see the telltale thin strings dangling after the larva emerge. Hope this helps. Thanks for your comment and question.

Reply by Marcia Morales on Sunday, May 31, 2020
Steve, That helps a lot. Thanks for your quick response and all your work for the monarchs!

Comment by Sarah on Sunday, July 26, 2020
 I've lost all of my mature caterpillars and chrysalis this year because of this fly. I'm not using any kind of cages or coverings. I think this is what I should go to to protect the caterpillars from this fly because none of them are surviving.

Comment by Win Heiskala on Monday, September 21, 2020
I just had approx. 8 butterflies emerge from the chrysalis stage, outdoors. This is the third round/batch of butterflies this year, starting in the Spring. I am in Imperial Beach, south of San Diego. We only have a very small growing area with half dozen milkweed, but we have had a few years of multiple batches of beautiful monarchs. This time, when going from caterpillar to chrysalis, several didn't make it due to obvious infection by the Tachined fly. I wondered also if they were safe in the chrysalis but all of the ones that made it to that form became butterflies and flew away, I assume healthy. They looked healthy and acted very normal during the emerging process and eventual flying. I saw the damaged ones on the wall with the tell tale string (we removed and destroyed them), but I have never seen any on the ground as others have reported. Perhaps out visiting squirrel(s) enjoyed them. My worry now is whether the fly has gone or whether there is a major infestation just waiting. I don't know if there will be another round of eggs this late. One year we did have a butterfly emerge in December just before Christmas. WH

Comment by Laurie on Friday, January 22, 2021
We have monarch caterpillars make it fine to chrysalis stage and die-sometimes parasitic fly larva drops out sometimes not. This fall after seeing many caterpillars on our large plants of milkweed and parasitic flies stalking, I started bringing every one inside that I found. Out of 31 brought in only 7 hatched from chrysalis-normally we have about 80-90% success, previously bringing them in in the spring to dodge ghekos! This year has been bad with the flies for some reason-I just ordered the traps and will be happy to get some off our planet! (We are north of Houston, Tx)
PS-One thing I started to notice, and began paying attention to is when some of the caterpillars made their J, I noticed a bit later their antennae would go limp...but they still made a chrysalis...but never hatched. I'm wondering if this is an indication of parasitic infection? I'm going to keep a record with our next batches.

Reply by Steve (Cranial Borborygmus)
Hi Laurie, Thanks for your comment. I've been doing this for years, and I've had good years with what seems like almost 100% success, and bad years with what seems like almost 0% success. My biggest problem has always been tachinid flies, but the fly population varies greatly season to season and I don't know why. I know there are other problems that people report as well. I've never heard of geckos being a problem, though we do have blue bellied lizards and I've often wondered if they might eat Monarch caterpillars.
As for the limp antenna just before the caterpillar turns into a chrysalis... that is perfectly normal. I've watched several caterpillars turn into chrysalises, which is one of the coolest things I've ever seen. But you have to keep a constant eye on them to actually see it happen, because if you stop watching for even 15 minutes, you can miss it. Since it's hard to watch them all the time after they go into a J, one of the clues that they are getting ready to turn into a chrysalis is that their antenna go limp. Again, that is normal. In any event, sorry to hear that this was not a good year for your caterpillars. But perhaps the new year will be better! Good luck!

Comment by Shirley Weismann on Sunday, September 11, 2022
I have been raising Monarchs for over 20 years here in Illinois. I have experienced almost every disease and predator that can happen to Monarch butterflies and milkweed, but for three years I have been trying to find out what is causing me to lose from 100 to 300 caterpillars a season. The cats do very well up to their 2nd and 3rd instar and then their eating slows down, and I find them motionless for long periods at a time. Eventually the caterpillar would starve to death. It looked small but perfect and eventually deflated and turned a rust color. I never euthanized them because they could have been molting. It really isn't necessary. The T fly is not contagious, and the larva and pupa is easily found and destroyed. I only brought in Monarch eggs and a few newly hatched caterpillars. Eventually I could not get one caterpillar to live to its 4th instar. I did consider the Tachinid fly in 2019 but there wasn't any mention of the T fly laying eggs in Monarch eggs. I am researching again and found that it is a possibility, but it hasn't been proven.
My theory is that if the T fly laid its egg in or on the Monarch egg, the little caterpillar would die before the T fly larvae would mature and that is why I never found any outside damage to the caterpillar. I dissected one and there wasn't any sign of larvae. The only thing I can't answer is that if I am right, why would a T fly lay its egg in a host insect that would not live long enough for the larva to mature?

Reply by Steve (Cranial Borborygmus)
Hi Shirley, Great comment. Like you, I've read speculation that Tachinid flies sometimes lay their eggs in Monarch caterpillar eggs, but I've never seen any proof of that, and like you say, it doesn't make sense to me. I think you're dealing with something else. Unfortunately there are other issues that Monarch caterpillars can face. And like you said, if it was Tachinid flies that were killing your caterpillars, I think there would be evidence left after the caterpillar dies and the Tachinid larva eventually emerge.


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