From mid-August through the end of the summer is when the fall webworm is noticed on walnut and other hardwood trees. This insect is easy to identify by the loose, gray, silken tent spun by a cluster of caterpillars feeding on the leaves at the end of the branch. The caterpillars are tan to yellow in color, hairy and up to 1 inch long.
Fall webworm tents start small, but the caterpillars enlarge the tent every few days as they grow and consume the leaves within the tent. By the end of the summer tents may be 2 to 3 feet long and enclose the entire end of a branch. The old gray webs hang on the trees most of the winter.
Although the fall webworm has been recorded feeding on more than 200 species of deciduous trees and shrubs, the favored host in Iowa is the black walnut tree. Tents are particularly common on walnut trees growing in the open or on trees at the edge of the woodland.
Damage caused by the fall webworm is not significant to well-established, otherwise healthy trees. Damage is more unsightly than serious because of the limited amount of foliage consumed and the time of the year. Trees are not killed by this pest and control is not essential. Applications in very late summer or early fall (later than mid-September) especially, are of no benefit and should be avoided in order to preserve predators, parasites and other biological control organisms.
Pruning to remove webs when they are first noticed is one possible control. Prune and discard the webs you can reach and disregard the rest. Because caterpillars remain in the tent, time of day is not important. Insecticides labeled for caterpillars can be used for control but they must be applied with sufficient pressure to penetrate the silk tent and reach the leaves within. For more information on insecticides please see this article.