Fall Webworm

This pest is native to North America and is common from Canada into Mexico. It is one of the few American insect pests that has been introduced into Europe and Asia. Its hosts include more than 85 species of deciduous forest, shade, and fruit trees, with preferences varying from region to region.

The fall webworm is most often discovered when the unsightly, light gray, silken webs are observed on the trees in late summer and early fall. Webworms enclose leaves and small branches in their nests, unlike tent caterpillars which make a smaller nest in the crotch of branches. The large silk webs enclosing tips of branches are sure signs of fall webworms. The caterpillars remain inside the webbing and if food runs out new foliage is encased.

Description and Life Cycle

The caterpillars are covered with long white to yellowish tan hairs. Two races of fall webworms occur in North America: the blackheaded race light greenish-yellow to pale yellow caterpillars with two rows of distinct black tubercles; the redheaded race is more tan in color with orange to reddish tubercles. The caterpillars make distinct jerking movements in unison if the nest is disturbed. Adults are about one inch long and range from pure white to white with a few black spots.

The fall webworm overwinters in the pupal stage. Adults emerge from late May into July. The eggs are usually deposited on the undersurface of leaves. They hatch in about one week and the small mass of caterpillars web over single leaves and feed,thereby skeletonizing them. As the caterpillars grow they web over additional leaves and finally are able to eat entire leaves. The larvae mature in about six weeks, at which time  they drop to the ground to pupate. The number of generations per year varies according to the region. The main generation occurs during July and August in the Northeast.

Damage

This pest usually eats leaves late in the season and the nests are generally concentrated to limited areas. Because of this, little real damage is done to most trees. However, the nests can look very unsightly and multiple generations in long summers can lead to significant defoliation.

Control

Though the webs are very unsightly, damage to most trees is considered to be insignificant. On small trees, nests can be cut out and destroyed. Soap or other insecticides may be applied from mid- to late- summer. If spraying is necessary, the bacterium Bacillus thuringiensis (B.t.) is the least toxic of the effective materials. Several more toxic chemicals are registered and available for homeowners for this pest. Applications can be made when larvae arefirst noticed.

Adapted from: Cornell Cooperative Extension; Virginia Cooperative Extension, 1999; Ohio State University Extension