During a break in this afternoon’s rains, I was walking along our street checking for migrant birds. Something caught the corner of my eye — I looked down and found this:
We find at least one of these caterpillars each summer, always on the street or sidewalk under an American Elm tree. The green color, overall shape, and especially the pointed projection on the tail convinced me that this must be some kind of hornworm — the caterpillar stage of a sphinx moth.
The caterpillar was motionless. I wondered if it had fallen 30 or 40 feet from the tree above and died. Just in case it was merely stunned, I brought it in our house and put it in a plastic box with some elm leaves. When Ethan and I checked it a few hours later, it was moving its head slowly, stiffly swinging back and forth, so it was still alive. Maybe we could save it!
Then we had a brainstorm. We knew that many hornworms dig into the ground and make a pupa — the transition stage from caterpillar to moth — without spinning a cocoon. And caterpillars often stiffen up before they split their skins, revealing the pupal stage within. Maybe this caterpillar had dropped to the ground on purpose, but had the bad luck to land on the street rather than soft soil. So we added some damp sand to the box, set the caterpillar on the sand, and waited.
Two hours later Ethan checked the box — the caterpillar had disappeared! However, a bit of digging revealed that it had merely dug its way into the sand and curled up into a tight C-shape. We left it alone, because pupating caterpillars can get all messed up if you bother them during this critical transition.
We’ll check again tomorrow and let you know what happened.
Update: The caterpillar continued burrowing in the sand, digging all around the container and finally settling into a rounded cavity just below the surface. Then, after about a week, it died without making a pupa. It turned out to be a tragedy after all. We were very sad.
Follow these links for more information about sphinx moths:
- Hornworms and “Hummingbird” Moths from Colorado State University Extension
- Elm Sphinx (Ceratomia amyntor). This web page shows the Elm Sphinx as having four horns — the one we found only has one. So, even though it seems to eat elm leaves, it must be a different species.
- Twin-spotted Sphinx (Smerinthus jamaicensis). The caterpillar shown on this page is a better match for ours, although not a perfect one. However, the text says the “larvae can show considerable variation.” According to the Colorado State University Extension site, some Twin-spotted Sphinx caterpillars eat elm leaves.
If our caterpillar pupates successfully we’ll have a sphinx moth to identify, which may be easier than identifying a caterpillar or pupa.