I spend a lot of time peering at branches in the winter. Although most of them are inevitably bare except for the spring’s half-formed flower buds, occasionally you’ll turn up something really interesting. I also like to do my annual winter tent caterpillar egg check (so far I’ve found two trees with egg cases). Last week, while out peering at branches, I found a few unidentified galls, a couple of things that looked like they might once have been alive (or maybe not), and this, which was about 1 cm (3/8″). This was the only thing that I knew what it was when I saw it, but only because I’d found some just like it last winter. At that time I didn’t know what they were, and I’d posted the photos to BugGuide.net for some help in identification.
It’s the empty, hardened skin of a caterpillar that was parasitized by a wasp. The wasp was a member of the genus Aleiodes, a group of ichneumonid wasps who specialize in parasitizing caterpillars. Their larvae burrow into and live inside the caterpillar’s body, feeding on the caterpillar from the inside. They affix their dead host caterpillar to a branch, to make sure it doesn’t fall off, by chewing a hole in the belly and secreting a glue-like substance. You can see the “flaps” of glue securing the caterpillar more clearly in the photo below. The skin of the dead caterpillar dries out and shrivels, becoming a mummified version of the living thing. The head often falls off, leaving just a rounded lump that doesn’t really look like a caterpillar anymore, and could be mistaken for a bud or gall. The wasp pupates inside the mummy, and the adult wasp emerges by chewing a hole in the shell. All that’s left, come winter, is the mummy, still securely attached to its branch.
When I found the mummies last year, I was directed to this online resource, which is basically a field guide to Aleiodes wasps and their mummies. Based on that reference, I think this might be some species of Noctuid moth, parasitized by the wasp A. terminalis. This wasp is very common and widely distributed across much of the continent. It generally has four generations a year, with the fourth overwintering inside its mummy (so if you find one without a hole, chances are it’s still got the wasp inside).