Maybe you've seen those tents of webbing on the ends of tree branches and wondered what creature made that elaborate structure. Well, wonder no more! The fall webworm, Hyphantria cunea (Drury), is responsible for building those tents. The common name is a bit misleading. For one, these are caterpillars, not worms; they're the larval form of a moth. Also, these caterpillars can be seen building their webbed tents not just in the fall; these little guys are active from July through September.
So what exactly are those caterpillars doing in that tent on your tree, and is the tree being harmed? Fall webworms build these webbed nests to protect them while they feed and grow. While they completely defoliate the branches within their nest, this doesn't harm the tree. Fall webworms are normally found in deciduous trees; they're eating leaves that were just about to fall off the tree for autumn. So while their nests may be a bit unsightly, the only damage these caterpillars cause is aesthetic.
The mature caterpillars are covered with non-stinging hairs and can be either a lime green with black spots or sometimes a darker colored body; their heads are either red or black. Fall webworms overwinter in crevices in trees or leaf litter, emerging as white moths that will lay eggs from May to August.
These caterpillars are not picky eaters; they have been known to feed on 85 species of trees. Some of the host plants on which you'll most often find them include hickory, pecan, persimmon, sweet gum, black walnut, and bald cypress.
Trying to control these caterpillars is likely to cause more harm to the tree or your landscape than the caterpillars themselves. Pruning off areas where they are feeding can cause unnecessary damage to the tree, since the caterpillars are only eating leaves which will regrow in the spring. Spraying is not warranted since the caterpillars will not cause any lasting damage to your tree, and isecticides can interfere with the natural predators and parasites that feed on fall webworms.
So while you may not love the look of fall webworm nests in your trees, try to remember that before you know it, they will be out of your canopy and bugging you no more.