Suicidal Caterpillar Infested with Tachinid Flies
I thought this one would be a good lead-up to halloween, for reasons that will become obvious.
I spotted this caterpillar on June 4, 2012 in an odd location: it was on a rock in the little stream that runs beside the road we live on, with its head immersed in the water. It looked for all the world as if it was trying to drown itself. It had an oddly bloated, sausage-like appearance, but wasn’t completely dead, so I brought it home to see what was going on with it.
I admit that a big part of my reason for bringing it home, was that I suspected that it might not be what it seems. See, I have this hypothesis that caterpillars that are out-of-place and doing things that appear suicidally dangerous, really are trying to kill themselves. What’s more, I suspect that a big reason why they would be doing this, is because they are fatally infected with a parasite. And even though it is too late to save themselves, if they can get themselves killed they can keep their parasites from hatching, and so save their nieces and nephews and cousins from getting attacked and infected by them later.
I originally thought that the white objects glued to the front of its body were the parasites, but it turned out they weren’t. They never did anything other than hang around. I don’t know what they were.
It lived for some days, but didn’t act as if it were well. It refused to eat any leaves that I gave it, and barely moved. Finally, one day it died, and on June 9 these maggots cut a hole in its side and crawled out
Two of the maggots came out right away, and a third just stayed inside with its head(?) poking out. At this point, the caterpillar looked pretty much deflated.
I put the maggots into a jar with a moist paper towel, and they promptly made pupae like this one.
that hatched out to produce these flies on June 22. They looked a lot like a standard housefly, except they were only maybe 2/3 as long as a housefly.
This is one of the hundreds of species of tachnid flies. I think I can narrow it down to one of the “red-tailed tachnid flies” in the genus Winthemia. Partly based on the reddish tip of its abdomen, and partly because it is apparently the most common genus of tachnid flies in North America. And they parasitize armyworm and cutworm caterpillars, which these appear to have been doing.
 Behaviors that I consider signs that a caterpillar is trying to get itself killed: (a) normally well-camouflaged caterpillars that have moved to a place where their camouflage doesn’t work, as if they are begging to be eaten; (b) caterpillars eating toxic plants that aren’t listed as being among their normal foodplants; (c) crawling someplace where they are likely to either get drowned, or dried out and cooked by direct sunlight. Of course, depending on the parasite, some of these actions could be things that their infection is driving them to do. For example, a multiple-host parasite that alternates between infecting caterpillars and infecting the birds that eat them could drive them to expose themselves to predators. Or parasites with an aquatic phase might drive them to drown themselves. But, and this is a key point, whether they are doing it to kill the parasite, or the parasite is making them do it, in either case the parasite is there!
 Or maybe they were. Some tachinid fly eggs look an awful lot like these. It is possible that the white objects are either eggs from the original tachinid fly that never hatched, or from a later tachinid fly that didn’t realize the caterpillar was already infested. Or, maybe empty eggshells look about the same as unhatched eggs?
 Fly maggots don’t really have a distinct head. One end has a mouth, but other than that it is really hard to tell the ends apart. It is quite possible that what we see here is actually the rear of the maggot, and that those circles are its breathing spiracles. It would be pretty hard to breathe inside of a caterpillar, after all.