Last week, while checking on our prairie, I noticed a couple big congregations of ‘woolly bear’ caterpillars. I found several clusters of hundreds, which was impressive. They didn’t seem to be tied to any particular plant species or habitat structure – just apparently random groups. The numbers were impressive.

Yesterday, I was back out at the prairie to catch the sunrise and I saw the caterpillar clusters again, but this time, there was a stark difference. Many of the caterpillars were way up high in the canopy – at the tops of grasses, forbs, and even fence posts. And many of those high elevation caterpillars were dead. Some were still intact, but others were withered or empty-looking shells of their former selves. The calm winds, dewy conditions, and warm-colored sunlight made for some great photography, but I spent much of the time distracted by the mystery – why were all these caterpillars climbing up high and dying?

I believe the caterpillars I saw were larvae of the Virginian tiger moth (Spilosoma virginica). The species can be really variable in color, so while I found both pale creamy-colored caterpillars and almost bright orange caterpillars, they could still all be the same species. (It’s also very possible that there were multiple species – I’m not an expert on moth caterpillar identification!)

The climbing behavior reminded me of stories I’ve read and heard about insects that are attacked by parasitoids, fungi, or viruses that alter their behavior before eventually killing them. In the case of some fungi and viruses, insects are compelled to climb up high before dying, which helps the spores of the infecting agent spread widely after the larva dies. It’s certainly possible something like that is at work here, though the appearance/condition of the dead caterpillars I saw didn’t match the kinds of descriptions I found during an online search last night.

Parasitoids seem like a likely possibility – either flies or wasps. Stephen Spomer of the University of Nebraska-Lincoln says he has seen similar behavior in the past with tiger moth larvae and has reared parasitoids from them (entomologists are the coolest…). However, neither of us could explain why parasitoids would stimulate climbing behavior. Stephen suggested there could also be multiple factors at work (parasitoids and fungi, for example). If so, that’s pretty rough for those cute little caterpillars.

I did see some caterpillars that were still acting normally – crawling around on plants and feeding. They were also down at a more reasonable elevation in the vegetation, rather than at the pinnacle of the highest plant they could find. I hope they escaped whatever afflicted their colleagues and weren’t just in an early stage of infection that hadn’t fully kicked in yet.

I’d appreciate hearing from anyone who knows more about what I was seeing and can provide more explanation. I love a good mystery, but I love fascinating natural history stories even more. There’s got to be at least one good story here and I’d really like to hear it. It’s easy to feel badly for the caterpillars, and that’s certainly a reasonable emotion. At the same time, parasitoids and other possible infection agents also have an important role to play in controlling the populations of insects like this. I think it’s possible to feel bad for individuals while appreciating the broader picture that led to their affliction (even if we don’t yet understand that picture fully).

Tiger moths have been around for a long time and are likely to continue existing long into the future. I’m sure they’ve been dealing with whatever parasitoid or other organism is attacking them for a very long time. I’m fascinated by what I saw yesterday and would like to learn more about it. I also wish both organisms luck. The caterpillars that survive have a long winter ahead of them and the mystery organism will also have to get through the winter so it can continue its own life cycle next year. Complex interactions keep prairies healthy and resilient. Carry on, little friends!