Unknown Monarch caterpillar illness

Hello all, I’m new to the forum but hoping someone can help! I am raising monarchs in my home and have had a pretty good run of it so far. However, I had an instar 2 caterpillar who became obviously ill and I had to euthanize him on 8/14.

He seemed to be having issues walking when I first noticed his odd behavior on 8/13, and it appeared to me as if he had something attached to his rear prolegs. With tweezers, I gently pulled what appeared to be plant material off, but saw no improvement. He started lying on his side and wasn’t eating, moving very slowly. I left him overnight to see how he’d do, but by morning on 8/14, it was obvious that he was not going to pull through. He hadn’t eaten overnight and was curled up off his leaf, barely moving. It seemed to me that there was something on his prolegs, but I was unable to tell what it was until after I euthanized him in the freezer and was able to take a good look with a camera and magnifying glass.

Anyone know what this is firstly, and what its cause may be? It looks to me almost as if there is some sort of tissue between the prolegs, possibly an injury of some sort or, perhaps, cringe the work of a parasitoid or some other predator? For the record, this little guy (caterpillar 25) was found as an egg in an untreated field. He progressed normally until this mystery malady took him. I wash my milkweed and keep caterpillars in separate, other-bug-proof containers until they reach instar 4. Thanks in advance for any and all advice!

The individual in those photos is clearly molting based on the head capsule. That would explain the poor grip and lack of feeding. The photo quality is too poor for me to make out what is on its prolegs, but if I were to guess it is either clinging onto the silk mat it spun as it was preparing to molt or, if it was very close to molting, it is possible you could have torn the outer cuticle on its prolegs if you moved it off the leaf too forcefully. Obviously, molting and being diseased are not mutually exclusive though, so it is possible, but I don’t see any sign of disease from the photo and it is unlikely at this stage (most Lepidopteran diseases are typically asymptomatic until later instars unless you’ve got a severe outbreak or something).

As a general rule of thumb, larvae should not be disturbed or removed off the leaf while molting or there is a risk of mismolt or failure to molt altogether. Their grip weakens while molting and thus they rely on clinging onto a silk mat to anchor their body, allowing them to crawl out of their old cuticle. This isn’t much of a problem with later instar larvae or species with robust grip (saturniids and sphinx larvae molt just fine regardless). Young instar monarchs tend to have pretty weak grip and might have trouble.


I was also just recently attempting to raise a few caterpillars from my garden, and unfortunately both of mine died within hours of each other by a mystery illness as well (both were at least 4th instar, raised from egg), maybe someone here can also shed some light. They began regurgitating copious amounts of green liquid which I can only imagine was leaf juice, and then would writhe around slowly for hours before succumbing. It was very saddening to lose them both. I ended up determining that they both ate a small amount of the same leaf and suffered these symptoms just a couple hours afterwards. I can only presume the leaf was contaminated with pesticides? The location from which I got the leaves wasn’t someplace I expected there to be any pesticide use so it was a real surprise. Do these symptoms sound right for pesticide poisoning?

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Hi, thanks for your reply!
Yes, I agree – he was definitely trying to molt, but while that explains some of his behavior, it definitely doesn’t explain all of it. I am pretty familiar with the behavior of molting caterpillars, and this was far from typical.

Let me be clear – I didn’t at any time remove him from his leaf. When I discovered him on 8/13, he was attempting to walk, albeit somewhat weakly, and was rolling over on his back. He was indeed due to molt, having been an instar 2 caterpillar for 3 days, and I was expecting to see the normal signs of molting. His behavior surprised me because of this. On closer inspection, he appeared to have a tiny bit of plant matter stuck to his back prolegs, and I was able to remove it without causing injury to him (I have a good magnifying glass and was able to do this very slowly and carefully). At the time that I removed the plant matter, whatever is visible in the attached photos was already present on his prolegs as well.

As far as I’m aware, molting starts at the head and the larva wiggles out from head down…please correct me if that is misinformation! As such, I wouldn’t expect to see molting activity along the legs until the molt had begun at the head, which it hadn’t yet (but as you noticed, it was close!).

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First off, welcome to the Forum. The behaviour you describe does not seem to me to be associated with moulting. I can only say, having reared a lot of Noctuid larvae in a past life, that strange things can happen to larval insects. Parasites usually show up in later instars, as do viral diseases. This could just be a malformation caused by hatching or the first moult. There are a lot of complicated things that need to happen correctly for a larva to hatch and complete development. It could just be some sort of genetic ‘glitch’.

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Hi Molly, I have never seen this personally, but I think you’re describing something consistent with pesticide poisoning. They didn’t happen to turn to black goo afterwards, did they? I hate to even invoke the idea of NPV, but from what I have read, you are describing the onset pretty accurately.

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Hello, and thanks for your reply! I suspect you are probably correct as to what caused the poor little guy’s woes. I wondered about a genetic issue preventing or altering the molting process, but wasn’t sure how common that was. It is one of the only things that makes sense to me at this point. Thanks again!

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If I may ask, what is NPV? The caterpillars did not become gooey after death, just became a little shriveled/saggy like I’d say dead soft insects do. The leaves were from my neighborhood communal backyard so if there’s pesticide use I’m not aware of, I’d be rather concerned.
(Does washing the leaves actually help in these instances? I would think pesticide residue wouldn’t come off so easily, otherwise the leaves would be fine after the many rainstorms we’ve had.)

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NPV stands for nuclear polyhedrosis virus, aka black death. It is a devastating disease that is fatal and pretty ugly. More info here:

Sounds like it wasn’t likely NPV. As far as washing leaves to prevent pesticide poisoning, I’m not sure of the efficacy. I do know that you can wash off/ kill OE and NPV if you use a low concentration bleach solution on the leaves and rinse thoroughly after soaking.

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Thank you for the resource, it’s a very useful list of diseases. I’ve never had good success with raising leps, and I think I’d rather just leave it to the experts!


You’re welcome! I agree – it is much more involved than an unsuspecting layperson (such as myself!) might assume. In spite of a few hiccups, we have really enjoyed it. Our first monarch eclosed today (she’s my profile pic) and it has been amazing and rewarding to watch the whole process unfold on my desk and bookcase in the study. All the best to you and any leps remaining! :grin:

I take you have doing raising monarchs at home and are well-versed with their general life cycle. I haven’t raised Monarchs but have experience of last two years raising several butterfly and moth caterpillars. Many caterpillars sometimes die abruptly during growth, the reasons could be developmental defects, being infected by parasites like viruses, other microbes and wasps. There is also a stress of captivity, temperature/light changes, moisture, lack of fresh food, cage not cleaned etc.
In captivity, sometimes caterpillar can’t lose their shed skin properly, if we disturb them or try to pull it off…it affects their well-being. Better leave them on their own, they do fine without us meddling :). Just to aid the process, minimize use of tissue paper/cotton or use the ones that don’t release fibers. Also putting whole twigs/small branches instead of plucked leaves helps as it allows them to use the wood to rub their shed skin off.
I lost 2 Mormon caterpillars, around 5th stage where they undergo a major metamorphosis. Also minor changes in diet used for feeding are important (Different plant, frozen leaves, leaves from twigs kept in water etc.). They also need sufficient warmth/exposure to sunlight. I take them out of cage once in a while in early sunlight and put them in open container for a walk…and that helps a lot.
Also keep them safe from insecticides, once we had our neighbors doing pest control for termites and I lost half of my caterpillars next day.

Despite our best efforts some of them will still die and they do even so in nature…its just the process to keep their numbers in check. I once had 40 eggs of Tawny coster butterfly on a passionflower, 30 larave hatched, only 6 reached maturity…all outside, no intervention.

Lastly, if you are getting this on a large scale and suspect a viral/parasitic disease that could spread locally, check your plants/surroundings for signs and more caterpillars to reach a better statistical conclusion. There are lot of Facebook groups active on Monarchs, many of them have university scientists as well.

Thanks and best wishes

and for the caterpillars I have observed


These folks have some helpful advice (if you are on FB) https://www.facebook.com/groups/caterpillarrg/files

Hi Guarav,

Thank you for your reply and the benefit of your experience! While I am new to raising monarchs, I have had the benefit of a great deal of education on the matter prior to starting the process. I have a neighbor who has been raising them for some time who has been a great resource, as well as several YouTube personalities who post extensively about how to care for Monarchs at each stage.

I am also a registered nurse, and I have found that most of the basic things needed to support life for human patients (water, good food, physical activity, a safe amount of sunlight, a clean environment) are also, not surprisingly, essential for raising healthy caterpillars. I do not tend to handle my little charges unless they appear to be ill, in which case (depending on stage of development) I may gently investigate to see if there is some issue i can help with. This is rarely warranted. I feed them minimally twice a day (with the bigger ones, this is often 3-5 times a day as their appetites take off!) and clean their cages of frass at least once daily. I have several different sizes of containers for them, which allow them to move and explore as they grow in size. I handle them minimally in an effort to combat the stress of captivity, and this strategy seems effective. I do my best to keep some semblance of a circadian rhythm by minimizing or eliminating artificial light in the evening and maximizing indirect daylight and artificial light during the daytime hours.

All that said, my unfortunate caterpillar in my original post appears to be an isolated incident. Everyone else seems fine and healthy, and I suspect that, as a previous poster indicated, there may have been a genetic or hormonal issue that prevented him from completing his molt.

In spite of losing a few caterpillars over the past month, we have been met with great success. We had one monarch eclose today, a boy, who my son named Thomas. He’s a beauty!

Thanks again to all of you for all your input!!


That’s great news…Wish you all the luck…Developmental defects are natural, I had a female moth yesterday which was was missing a leg, when I found it eclosed the morning it seemed nearly incapable of flight and crawling, but she gained strength by evening and was taking small flights. As I couldn’t release her in open outside, I took her to the garden and left her on a nearby host plant. The sight also has a street lamp above it and individuals of the same species spotted around. I’m sure she’ll do fine :)

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Thank you! Congrats for successful eclosions! I have had much better success rearing hemimetabolous insects (I raised Scudderia katydids for several years with no mortality), but those darn soft, vulnerable larvae… I have mad respect for anyone who is able and willing to dedicate so much time and resources to rearing leps, especially for species that are imperiled or those we lack life history knowledge of (e.g. many gall-forming insects, or others where host plant isn’t known or larval and adult stages haven’t been linked).

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