Monarchs grow fast and need a near-continuous fresh supply of milkweed to eat. Milkweed produces a poisonous sap that the caterpillars ingest when they eat the leaves. This then makes them taste bad to potential predators, serving as their primary line of defense. Like I said in my last blog, unless you have easy access to milkweed, I wouldn’t recommend taking monarch eggs home, or you’ll be struggling to feed them.
Luckily, there’s a field near the office that grows milkweed. Every other day or so I would snip off some leaves, rinse them in cool water to wash away any unwanted pests, and place them stem-side down in a bit of water inside a Tupperware container for my caterpillars to munch on. At this stage in their life, the larvae are true babies- they basically only eat, sleep, poop, and grow. And for their size, they do all four remarkably fast.
Until one day I noticed that the oldest of the caterpillars had settled on a leaf and stopped moving. I’d been watching them pretty closely for almost a week at this point, so I knew how ferociously hungry they were. It made my heart sink to see one of them seemingly frozen. Surely something must be wrong. Had it gotten sick? Or perhaps taken on a parasite? I wondered if I hadn’t cleaned the Tupperware or milkweed leaves well enough. I remembered reading online that improperly cleaned or ventilated nurseries could allow bacteria to grow that can give monarch caterpillars life-threatening diseases. There was one in particular I was terrified of- the “Black Death”. Sometimes caterpillars will get sick and stop moving and eating, slowly go all dark, and eventually get mushy and die. In my opinion this was the worst thing that could happen to my caterpillars, so of course, I immediately became paranoid that that was exactly what was happening. But in fact, there are several threats that these little guys face, including being eaten by other animals and even becoming hosts to the larva of parasitic wasps (are there pests in your woodlot that could be harming your native plant and animal populations?).
None of what I was reading sounded reassuring, so needless to say, I was concerned. I even quarantined the one caterpillar in the hopes of saving the younger two from the same horrible fate. It felt like all my worries spearheaded to a point when I looked down at my immobile caterpillar and saw it had a strange new pile of black mush sitting right behind it. The caterpillars make a lot of waste, but this didn’t look like frass, or caterpillar poop. Convinced my caterpillar was now dead or dying, I went to the internet to look up this specific symptom.
To my complete relief, my caterpillar was fine. Better than fine actually- it was healthy and growing. What I had observed was a change of instar, or when a caterpillar molts and sheds its old skin so it can continue to grow. In the 24 hours before shedding the caterpillar will stop eating, attach itself to one spot and rest a bit before eventually crawling out of its old skin, which it then eats (this explained why I never saw molted skins to clue me in on what was happening up until this point).
Good news as this was, I did feel a little silly for jumping to conclusions and not seeing this information in my initial research on raising caterpillars. Learning this important part of a caterpillar’s life cycle was a bit of a “better late than never” kind of a situation, and for the next three weeks of caterpillar rearing, I was much more relaxed when I noticed the little crawlers took a break from their milkweed.