Adult Fall Webworm Moth

2015 January 10

There were several of these snow-white moths at the porch light on June 12, 2014. Some of them had bald spots,

And some of them didn’t. There was a little bit of orange hair on the underbody, and the eyes were black.

There may have been some color on the abdomen under the wings, but since they never opened their wings I can’t really tell.

Since these are clearly very common, I expect that they are probably adult Fall Webworms, Hyphantria cunea. They range in color from plain white like these, all the way up to white with prominent black speckling, and the orange on the underbody evidently also ranges from minimal to very pronounced. Their caterpillars are well-known for infesting most of the common shade trees and fruit trees, and in fact I’ve photographed the caterpillars on several occasions. The caterpillars make silk webs similar to what the Eastern Tent Caterpillar make, except that while the tent caterpillars tend to make their webs in crotches of the trees in the spring, the Fall Webworms make their webs on the branch tips in the late summer and fall. While the fall webworm nests are pretty unsightly, they don’t hurt the tree much, because by the time the webs get large the trees are almost ready to drop their leaves and go dormant for the winter, anyway.

I’m not really sure whether the white is a camouflage color or not. They are certainly prominently visible on the side of our house, but their are a few substrates where they’d be well hidden. In particular, on the bark of Paper Birch or Aspen, both of which are nearly white. There are also some fairly pale lichens where they’d blend in, and they’d be pretty hard to spot during flowering season in any of the many trees that have white blossoms. On the other hand, it is just barely possible that they taste bad and the white is a warning color – since they fly at night, the traditional orange-yellow-black color combinations wouldn’t be very effective, but white would be reasonably visible.

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