Insect Explosion: Parasitoid or Bird?

When I first started to become interested in insects two summers ago, I had a lot of inspiration thanks to the Catalpa trees in my backyard. I've always loved Catalpas: their flowers bloom starting in June and are spectacularly beautiful. They also smell really great.

One of the insects that can be found on Catalpa is the Catalpa Hornworm (Ceratomia catalpae). This caterpillar in the family Sphingidae can grow quite large (~3 inches) and goes through boom and bust cycles. During the summer of 2010, it was definitely a boom year for this caterpillar, which provided me with ample specimens.

There are also ample caterpillars for parasitoids to have a field day. Braconid wasps are especially prevalent, resulting in many of the caterpillars dying as they become vessels for broods of wasps.

Here you can see a Braconid wasp crawling on the cocoons.

This was a common sight, so I got used to seeing it after a few days. But once I found one that looked dramatically different (even from the cocooned ones), I took notice.

Yeah, I'd say that is dramatically different.

So....what happened? Good question! This guy lost the lottery in a spectacular way. He just exploded and that was that. Obviously, this is not like the parasitoidism of the Braconid wasps. If I were to venture a guess, I would blame a parasitoid fly maggot bursting out of the body: you can see a trail of liquid that comes out of the caterpillar's body and curves down by its head that looks like it could have been made by a maggot dragging itself along the leaf.

I haven't researched fly parasitoidism enough to put too much confidence behind that hypothesis, however. It also could have been a bird or other predator that decided to have a little snack, but wanted to leave a warning for the other caterpillars.

I would love to have the answer to the question, so I'll have to check for more caterpillars this summer. Maybe if I'm lucky, I'll find a maggot this time around, or see something else that explains it.