Mummified hornworms

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.

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.

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.

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.

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.