Aug 16 2011
These are the communal webs of fall webworm caterpillars (Hyphantria cunea).
Their mother laid a mass of eggs on a deciduous tree, preferably a cherry, apple, ash or willow. A week later the eggs hatched into tiny caterpillars and they began to build their web, mostly at night.
The caterpillars live inside the web, molting as they grow, and extending it to surround the leaves they’re eating. The web can become as much as three feet long but it’s very different from the tent webs we see in the spring.
Tent caterpillars build in the forks of branches and come out of their nest to eat. Fall webworms build at the tips of branches and stay inside to eat, only emerging on very hot days (too hot to stay inside!) or when they’ve reached their final instar and are ready to pupate.
Hyphantria cunea overwinter in the pupal stage under loose bark or debris on the ground. When they emerge in the spring they look like this with a 2″ wingspan:
Unless you have an infested orchard, fall webworms aren’t that bad for trees and they’re quite good for birds.
The caterpillars don’t permanently harm the trees because they’re eating the leaves at the end of the growing season when the trees would drop them anyway.
And they’re an important food source for birds. Migrating warblers see the webs as huge advertisements: “Come eat! Good protein inside!”
When you see fall webworms in the trees, think “happy warblers.”
(Web photo by G. Keith Douce, University of Georgia, Bugwood.org. Moth photo from Wikimedia Commons. Click on each photo to see its original.)