Tay Meadows Tidbit – Caterpillar mummy

Mummified caterpillar shell, perhaps by A. terminalis

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).

Mummified caterpillar shell, perhaps by A. terminalis

Mummified hornworms

Mummified sphinx caterpillar shell

When out walking Raven the other day, I came across some little cocoons on a small sapling. Ordinarily I would probably have missed them, and I’m not sure why it was that the first one caught my eye. But I saw it, and stopped, and took some photos. And then I looked at some of the other branches on the sapling. After a moment or two I spotted another. And then another. I think my finally tally was 13.

Those will be the subject of tomorrow’s post, most likely, but the reason I bring it up is because that instance of being rewarded upon paying slightly closer attention, of discovering this stuff that I had passed by multiple times before and may have again had I not looked a bit closer, prompted me to pause and examine some of the branches I was walking by yesterday when I took Raven out. I didn’t know what I was looking for. Cocoons, maybe, galls, something abnormal or that showed that something had been there.

Mummified sphinx caterpillar shell

What I discovered was way cooler than anything I had envisioned finding. I had to post the images to BugGuide.net for a definitive ID. I got as far as thinking it was a caterpillar that had been parasitized, perhaps by a fungus, and the spike was the fungus growing out of it or something.

Turns out it’s the remains of a sphinx moth caterpillar, a young one that hadn’t grown very big yet. Many of them can get quite large, up to a couple inches or more. This was less than one inch long. However, the horn sprouting from its rear end is a characteristic of this group of caterpillars and not a fungal growth at all, an appendage that gives the group their “caterpillar name”, hornworms. But it had lost its head, and the caterpillar itself was long since dead.

Mummified sphinx caterpillar shell

The killer? A wasp in the genus Aleiodes, a group that bears the common name “mummy-wasps”. There are about 90 species in this group in North America. These wasps parasitize and eventually (but not right away) kill leaf-eating caterpillars, including many common and “pest” species such as Gypsy Moths, Tent Caterpillars, Fall Webworms, and others. The wasp larva develops inside the caterpillar’s body, eventually killing its host once it’s eaten enough, but it doesn’t consume the skin. Because the caterpillar would likely fall off the branch once it died, the larva actually affixes its host to the branch with a glue-like substance by chewing a hole in the caterpillar’s belly. When it pupates it leaves through a hole in the back of the dead caterpillar’s shell, but the caterpillar itself usually remains identifiable. This is unique among parasitoids, most of which consume the whole body, or the body otherwise becomes shriveled beyond recognition.

Mummified sphinx caterpillar shell

BugGuide.net pointed me to an identification guide put out by the US Forest Service on the common eastern Aleiodes species and their mummies. Based on this, I think my caterpillars were Waved Sphinx moths (a species I have actually encountered at my parents’ old place), and the guilty party therefore A. ceratomiae (a species I have not encountered, at least not consciously). This photo from Wikimedia Commons is of an Aleiodes (not A. ceratomiae, but similar) parasitizing a Gypsy Moth caterpillar.

Mummified sphinx caterpillar shell

In the case of the caterpillars previous, there was no exit hole. Chances are that the wasp larvae in these caterpillars died before it fully developed. I found two of those. I also found four of these brown empty cases, as in the photo above. Turns out, they’re the same thing, only the wasp larvae made it to maturation. The hole is the exit where the adult wasp left. The horn of the caterpillar at some point fell off during the drying process (as the head had done much earlier on both the above and this), but had been affixed to the right of the hole in this photo. You may note that this one is smooth, but the others are kind of spikey. The person at BugGuide.net who ID’d these wasn’t sure whether that was just the natural mummification process, or if perhaps there was also a fungus involved.

So there you go. You never know what you might find if you peer closely.