May 26th, 2007
raise the alarm!
I believe the average person’s reaction to seeing an Eastern Tent Caterpillar nest may be to become grossed out, or perhaps angry and determined to rip it down. I’ll admit that I experience some of that urge to rip down nests when I discover them in our apple trees. In that case, I generally cut away the branch with the tent and toss it across the drainage creek where the caterpillars can do no real harm. However, when the caterpillars are in just about any other kind of tree, I leave them alone to do their thing. The truth is, they don’t really do much harm, and are actually an important part of the food chain. Some birds such as the cuckoo, feed on tent caterpillars. Also, a number of parasitic flies and wasps lay their eggs on the caterpillars. As is the case with many insects, and particularly caterpillars, it’s a bug-eat-bug world out there and few survive to adulthood.
Normally, I don’t pay tent caterpillars a great deal of attention, but the above nest caught my eye when Sabrina and I were out for a walk on Thursday afternoon. On the southwest side that was receiving the full afternoon sun, there were few caterpillars (see above – click on image for larger view). However, the shaded northeast side was crowded with caterpillars. These communities of caterpillars use their nests as a shelter from predators (birds and insect parasitoids), but also for temperature regulation. They spend much of their time inside the tents between forays for food, but also move about on the surface of the nest to take advantage of sunlight and shade. There’s quite a lot of interesting information on the natural history of these caterpillars and particularly on their thermoregulation on this wikipedia page.
Okay, so why was I attracted to this particular nest of tent caterpillars?
Curiosity! As I was passing by, I happened to notice that the few caterpillars on the sunny side of the nest, were thrashing frantically in a typical caterpillar “alarm” mode. From a distance, I couldn’t see what was making them so worked up, so I decided to approach and see what was going on.
Now, I’m not alone in my curious streak. It definitely runs in my family. One of my favourite childhood stories about my youngest brother is of finding him standing in the middle of the living room rapidly twirling a dayglo type yellow or pink eraser on the end of a very long string. I asked what he was doing, and he said he’d discovered something weird and that I should watch the eraser. In seconds, a fly of some type blasted through the air and gripped the eraser and rode along as it spun in huge circles. My brother would stop spinning the eraser and brush the fly off, and then repeat the process once more. The fly would immediately race from wherever it was perched and grip the eraser again. I’ve often wondered how my brother, who was probably about ten at the time, discovered that flies were attracted to dayglo erasers. Go figure.
Anyhow, back to the tent caterpillars. Yes, I had to investigate the reason for the violent thrashing. I approached the nest with my camera switched on movie mode to see if I could capture some of the action. At first, I couldn’t find the source of the obvious agitation, but then I noticed a fly dancing back and forth across the nest (see still frame capture below). It would repeatedly rush at the caterpillars, but immediately back off when one of them thrashed its head wildly in the direction of the fly. I managed to capture a few sequences of this and have put one up online for those who are as curious as me. The short movie is in the usual .mp4 format, with no sound (approx. 900b). The fly is, in all likelihood, some species of tachinid. They are the bane of of many insects as they deposit eggs or almost-ready-to-hatch larvae on or very near to the intended host (see Stephen Marshall’s “Insects: Their Natural History and Diversity”, pg. 409, for a good description of the tachinid’s modus operandi). If you watch the movie, take note of the actions of the fly. It dodges in and out, trying to move near to the caterpillars on the exterior of the nest, but from what I can see, it also seems to be injecting eggs through the webbing of the nest when it is over a caterpillar that is concealed within.
So, you see — the usual suburbanite preoccupation with tearing down or lighting fire to caterpillar nests is probably unnecessary when you’ve got deadly tachinid flies taking care of business.
