While walking along the paths and trails here at the Museum, you may have noticed tiny green, brown or even black, wiggly objects dangling down from the trees on silvery threads of silk. Indeed, you may have walked into a few of them, smack in the face. They seem to be everywhere.
You may already be familiar with them, calling them inchworms, loopers, or cankerworms. You may even know that they are the caterpillars (larvae) of small unobtrusive brown moths. If that’s all you care to know about these little lepidoptera, you can stop reading here. BUT, I do have a lot more to tell you about them. So stick around.
As already stated, they are everywhere. They are consuming many kinds of tree leaves; maple, oak, and buckeye, to name a few. The damage is only too obvious.
The caterpillars even eat some of the flowers and buds.
They’re cankerworms, for sure. They’re variable in color ranging from light yellow-green to black. I had assumed they were all spring cankerworms (Paleactrita vernata). After all, it’s spring, and they are cankerworms. But are they really spring cankerworms?
There’s another species of cankerworm caterpillar in our area that’s active in spring, the fall cankerworm (Alsophilia pometaria). Fall cankerworms are very similar to spring cankerworms. They share the same variability of color. They eat, pretty much, the same trees and shrub leaves. They’re the same size, and they dangle from the trees where we humans like to walk. And, the females of both species are flightless, they CAN NOT fly.
There are differences though. The flightless adult female of spring cankerworms lay their eggs in crevices, under bark, or in other concealed places during late winter or early spring. They spend the first summer and following winter underground, emerging from pupation the following late winter/early spring.
Fall cankerworm females lay their eggs out in the open on twigs or branches during the fall. The adults of fall cankerworms emerge in fall or early winter after spending the summer (about four months) underground as pupae. The eggs they lay in fall overwinter, hatching about the same time as spring cankerworms, in late winter/early spring.
Quite by accident, I took a photo of what looks like fall cankerworm eggs while photographing a presumed spring cankerworm caterpillar.
After looking more closely at some of the other photos I took of the little larvae devouring our trees and shrubs here at the Museum, I noticed something interesting. There was yet another obvious distinguishing characteristic between the two species of cankerworm that I failed to notice before.
Fall cankerworm caterpillars have three pair of prolegs where the spring cankerworm has two (Prolegs are temporary legs worn by larvae of moths and butterflies, and some other species of insect, while in their larval state. They help the rather long and thin larval insect grip onto and crawl along twigs, branches and other substrate. The prolegs do not follow the insect into adulthood). Most, if not all of my photos showed caterpillars with three pair of prolegs. They were all fall cankerworms.
So, with the evidence of both prolegs and eggs, I’m convinced that we are looking at fall cankerworms dangling from the trees.
I suppose, in the broad scheme of things, it doesn’t really matter whether or not they’re spring or fall cankerworms. The leaves are still being eaten, the caterpillars are still hanging down from the trees and crawling all over whatever it is they land on when they come down to earth, and we’re still dodging them as we walk along the path, no matter what species they are. I was, nevertheless, very excited to discover they were the fall variety of cankerworm, and am glad I stopped to talk a closer look.
Oh, by the way, before you go about smashing the tiny caterpillars out of frustration at seeing so many of them, or out of terror if you have a fear of them, think about this. There are thousands, no millions, of neotropical migrant birds (like warblers, tanagers, and thrushes) heading our way, right now, from Central and South America. Those birds fly at night, stopping to rest and eat during daylight hours. They will eat those little larvae. In fact, you might say they depend upon them for survival.
What luck for a bird whose been flying all night to drop into a place like the Museum and discover cankerworms, everywhere cankerworms. They will devour the little caterpillars much like the caterpillars are now devouring the leaves, with abandon.
Have fun out there!