Learning about the monarch goes deeper than trying to figure out why they migrate to Mexico. It's not completely understood how a caterpillar even turns into a butterfly in the first place! Let's talk about the stages that lead up to their chrysalis.
Although I don't support The Dodo (it's a company by PETA), this video offers basic information about the monarch caterpillar! Below is a video I would have loved to embed on my website, but the uploader didn't make their video playable on other websites. Click the picture underneath to see the video I would have preferred to link! Very informative and basic.
For those who don't want to/can't watch videos, fear not! I can write it all out.
There are 5 instars in a monarch caterpillar. Instar is described as: "a phase between two periods of molting in the development of an insect larva or other invertebrate animal". If you're rearing a caterpillar you found instead of one you've helped hatch, it could be a bit difficult to figure out which instar they're in. Although I haven't done this myself, I've heard that looking at the length of their filaments (antenna) can be helpful!
During my journey of rearing Peanut Butter and Jelly, I've found that knowing what instar they're in isn't too important, but there were times here and there I was worried when they weren't moving or when they had a black-ish grey goopy streak coming off of them. But that just so happened to be them molting and shedding their exoskeleton (did you know they eat their shed skin?). Thankfully, the issues that could happen to my caterpillars didn't happen considering I reared them in a takeout ventilated container when they were small and then in a mesh enclosure when they grew up. Although they're in their chrysalis, that doesn't mean that they're out of the woods for problems that could occur.
I'm all for teaching people the ailments that could bring harm to a caterpillar, but I won't be posting pictures of any dead or dying caterpillars on my website. I will, however, link to a website that goes over them. This website does have pictures of dead or dying caterpillars. Below I will give descriptions of the issues that could overcome them.
Tachinid Flies (T-Flies): It's hard to tell the difference between a tachinid fly and a regular fly that's just there for your milkweed's nectar. If you can spot one as it's standing still and it has big red eyes and it's hairy, then that's a tachinid fly. These flies lay its eggs inside of the monarch. Once they hatch, it eats them from the inside out, showing no signs until the caterpillar is about to form its chrysalis. If you find yellow/white maggots on the bottom of the enclosure or dark red pupaes, then your caterpillar is sadly long gone. So how do you prevent this from happening? The prevention tips listed on the website linked is to only bring in monarch eggs for raising. If you do bring in a caterpillar, it seems that they can be infected at any instar, so just keep them separate and as safe as you can. If your caterpillar from outside does die from the tachinid fly larva, don't beat yourself up. You didn't do anything wrong.
Ophryocystis elektroscirrha (OE) parasite: This one is very hard to tell until it's too late. It's a microscopic parasite that comes off of the wings and bodies of adult butterflies. It can cause weakness, "disfigurement" (I don't like that word, but that's what the website says. If I find a kinder word I will use it), and death. The only precaution is to clean your caterpillar's egg, its food supply, its enclosure, and milkweed plant that you bring in. The easter migratory population has the lowest level of infection (8% of butterflies being heavily infected), the western migratory population has a moderate level of infection (30% of butterflies being heavily infected), and the non-migratory population in South Florida has the highest level of infection (70% of butterflies being heavily infected). Here is the website where I'm getting that data, caution, there are pictures of dead butterflies.
Nuclear polyhedrosis virus (NPV): This is commonly referred to as black death. The caterpillar will deflate, turn black and then liquify. It can also affect the chrysalis, turning it completely black. The prevetions include using a mesh cage (any other isn't recommended in the first place) for ventilation and proper water evaporation, cleaning up frass (caterpillar poop) and wiping the cage down (including keeping it dry) daily, rinsing the food supply with water before serving, caterpillars leaking fluid or refusing to eat for more than 24hrs should be separated from the rest, and disinfecting the enclosure and switching out the food supply if a chrysalis is affected. Here is more info on the subject; warning for dead caterpillars.
Pseudomonas bacteria: This is a bacteria that can be found in soil and on plants. It thrives in warm, moist conditions, which is why it's crucial to have a cage with good ventilation. This bacteria usually affects caterpillars that are already sick with another disease. The prevention tips are the same as the one for the NPV prevention tips, the only thing done differently for this bacteria is if you're using a potted plant for your milkweed, cover the soil or water it with a hydrogen peroxide mix to kill any pathogens.
Tainted milkweed: If your caterpillar has eated milkweed that has pestisides on it, your little one will start vomiting green. If this happens, it's recommended to rinse your caterpillar under a faucet and place it on a new milkweed source immediately. The reason we transfer them to a new milkweed is because the plant has already soaked up the pestisides and it is everywhere on the inside of the plant. Another sign of pestiside poisoning in caterpillars is dying in the middle of forming the chrysalis. Insect growth regulators, including organic pestisides like neem oil, can be the cause as well. The prevention tips are to grow milkweed without pestisides, find online milkweed resources that grow them without pestisides, realize that companies are not referring to caterpillars when they list their products as "safe for beneficial insects". No pestiside is a good pestiside for the insects.
Trichogramma wasps: Trichogramma wasps lay their eggs inside of monarch eggs. If your monarch egg turns completely dark (instead of only black on the top, that's the monarch's little head), monitor it for 48hrs. If it doesn't hatch, then little wasps may emerge in about 10 days. I'm not one to intervene with the insect's process of potentially developing, so even though the website says to smush the egg, I wouldn't because I don't know if the monarch caterpillar will emerge the day after the 48hrs or not. If anything, I would keep the egg separate if I'm not totally sure it's a trichogramma wasp infestation, and wait for someone to come out. The prevention tip for this pest is if you see a monarch depositing eggs, collect them right away.
Chalcid wasps: Yep, more wasps! I was a little shocked when I found out, too. The chalcid wasps usually wait for your caterpillar to j-hang and once your caterpillar is in its chrysalis, they go in and lay eggs inside of your caterpillar while it's still soft. The preventions is to use a mech enclosure. Too big of holes will let them in. If you're raising monarchs outdoors, don't leave the habitat open for long periods while cleaning and check the cage for flying guys before closing up the enclosure.
Anal prolapse: You can tell this happens when a bright green ball is exposed at the end of your caterpillar. It's not known what causes this and there's no way to prevent it.
Dehydration: Dehydrated monarchs can get stuck in their chrysalis, causing "deformation" and death. The only way to prevent this is to spritz the milkweed and caterpillars daily (this is why you need a mesh cage, mold can form and kill your monarchs and caterpillars). I personally don't do this, but that's because the milkweeds I have on my front enclosed porch are in filled waterbottles. I don't want to risk molding up and drowning my monarchs/caterpillars, so I just try and make sure things aren't too dry for them. If they are, I will definitely spritz them here and there.
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