Fall Webworm

2010 May 8

We were out walking in the back yard last August when we spotted this caterpillar on a leaf of one of our numerous juneberry bushes. Caterpillars are relatively easy to photograph, because they will generally sit still, so we just picked the leaf and brought it home for pictures.

It looked kind of like a relative of a tussock moth to me, particularly that sort of spike of hair on its rump, but I wasn’t having much luck finding a match for it.

So, I posted it on BugGuide, where Ken Wolgemuth set me straight.

It turns out that I couldn’t find it under tussock moths, because it isn’t a tussock moth. Instead, it’s yet another tiger moth – specifically the Fall Webworm, Hyphantria cunea.

The hairs are rather longer and sparser than I’m used to seeing on tiger moth caterpillars, most of which tend to be more on the wooly side, with hairs so dense that they obscure the skin. On this one the skin was clearly visible, and had a rather complex pattern.

The face was well guarded by long hair, giving it rather a shaggy-eyebrow appearance.

In many ways, they actually have a resemblance to things like tent caterpillars, even though they aren’t that closely related. In another case of convergent evolution, the fall webworms also tend to spin sheets of silk webbing around the leaves that they are eating to produce a shelter against weather and predators, much like the tent caterpillars do. There are two big differences, though: (1) fall webworms build their webs around branch tips, while tent caterpillars spin them in crotches of the tree branches; and (2) the fall webworms are most noticeable in the fall, while the tent caterpillars are most active in the spring.

The fall webworms don’t seem to be producing massive, tree-defoliating outbreaks around here yet, but they evidently can do that elsewhere (so we’d better keep an eye out for this happening here). On the plus side, they mostly don’t defoliate the trees until near the end of the growing season, when the trees are getting ready to drop their leaves anyway, so the actual harm to the trees is usually minimal. The webs are still pretty unsightly, though.

They have a very wide diet, it looks like they will eat leaves off of most of the common broadleaved trees, with a particular liking for the family that apples belong to. They also eat mulberries, which is a bit of a concern – we have mulberry bushes that so far haven’t been much affected by pests[1], but if these are becoming more common then that could change. The caterpillars wander away from their webs in the fall to look for a place to pupate for the winter, and then emerge in the spring as either unmarked snowy-white moths or as white moths with black spots. We’ll have to check for these around the porch light this spring. They are supposed to emerge as adults in May, so maybe we’ll see them any day now.

And, for a change, this is actually a native North American species. In fact, instead of being accidentally introduced here from Eurasia, it was accidentally introduced to Eurasia from here! It was found in Yugoslavia in the 1940s and spread through Europe, and later found its way to northern China and North Korea. So even though it’s an invasive pest species, at least it isn’t invading us this time.

[1] Mulberries are not exactly native to the area, the only way they get here is if they are intentionally planted. While they seem to do OK in the local climate around Houghton (which is nominally USDA Hardiness Zone 4), further from the lake it drops to Hardiness Zone 3, which is probably rather too much for them. And since they aren’t a really popular fruit tree locally (most of our neighbors have never even heard of mulberries), our three bushes are pretty much the only ones around for miles (there is one other that we know of in town, and that’s about it). As a result, the local insect species don’t seem to quite know what to make of them. Hopefully it won’t be too catastrophic when that changes.

3 Responses
  1. May 18, 2010

    I love the photo of the face. Do you ever look at your work and think of the biologists of the 1700s and 1800s who did everything with sketches?

  2. May 18, 2010

    Oh, certainly. I’ve tried sketching insects, and not only is it a huge effort compared to taking photographs, the results I get are not at all satisfactory. I can manage something that is recognizably, say, a beetle, but as for making a picture that could actually be used for identification, forget it.

    Sometimes I compare two general-purpose insect guides that I have: one from the 1940s with hand-painted illustrations, the other a very recent guide that uses photographs. Not only does the recent guide have probably 100 times more images, but the images are actually of what you see in the real insect. In contrast, the pictures in the one with hand-painted illustrations always seemed slightly wrong, and it was always hard to be sure whether the bug I had in my hand was really the same as the one in the picture.

  3. Ramsey Piotter permalink
    May 29, 2010

    I do like that one too

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