Jeffrey Hahn, Extension Entomologist
Photo 1: Fall webworms can detract from a tree's appearance but does little actual damage to it.
While most caterpillars are active during the spring, there are a few that are not feed until summer. Fall webworm caterpillars, Hyphantria cunea are first active during late July and can be found feeding into September. This caterpillar varies in color from pale green or yellow with two rows of black spots on its back with long fine white hairs.
However, an easy way to identify fall webworms is from the webbing they produce that covers the ends of branches. The caterpillars remain inside this webbing to feed on the leaves. The feed on a wide variety of hardwood trees (over 100 trees and shrubs), including black walnut, oak, birch, elm, ash, willow, cottonwood, and chokeberry.
Typically fall webworms attack large, mature trees and their feeding is minor and does not have any lasting effect on trees. Occasionally small trees are attacked; they can be severely defoliated and can even be completed encased by webs. The primary problem is to the appearance as the webbing can be unsightly. However, after fall webworms are done feeding the webbing eventually deteriorates and goes away on its own.
In the majority of cases, a fall webworm infestation can be ignored, especially if it is in a large, mature tree. Direct insecticide treatments are not effective as the webbing protects the caterpillars from sprays. It is possible to effectively spray fall webworms when the caterpillars first hatch and the webbing is still small, although people don't usually notice them then. If the webbing is within reach, it can be physically pulled out along with the caterpillars. Even if you don't get the entire webbing out, you can still knock many of the caterpillars out of the nest. Do not try to burn the webs; this is more harmful to trees than any control that is achieved.