Fall webworm caterpillars feed on the leaves in July and August. Immature larvae eat only the outer surfaces of the leaf and leave the leaf veins untouched, while mature larvae will consume the entire leaf right down to the petiole. The larvae feed inside a messy tent that they produce over the branches and leaves. As they continue to feed, they also continue to expand the tent to enclose more leaves. If a tree is heavily infested, it is possible to have several branches enclosed in webs. Given the time of year that the feeding takes place, damage is usually considered cosmetic. If the fall webworm has infested a small tree, some control measures may be warranted. Even though the late-season feeding damage is relatively minor, the presence of the messy tents can be disturbing to home gardeners.

Fall webworm overwinters just below the soil surface, or in leaf litter, as a pupa in a thin cocoon. The adult moths emerge from early to mid-summer. There are two races of this insect. The adult of the northern race is a white moth. The adult of the southern race is also white, but is often marked with wedge-shaped dark spots. Both races have a wingspan of about 1 ½  inches. The adults mate and the female moth lays egg masses of up to 1500 eggs on the lower surface of leaves of a host tree. The eggs will hatch in about a week. As they hatch out, the larvae quickly begin spinning their webs over the leaves on which they feed. This web enlarges to cover more foliage as the larvae continue to feed.

The larva of fall webworm is a caterpillar, about 1 inch long. A caterpillar of the northern race has a black head and pale yellowish-green body, with a broad, dusky stripe down its back. It has long, white to gray hairs that arise in tufts from orange or black protrusions (tubercles). The caterpillar of the southern race has an orange to red head and yellow-tan body. The tubercles are orange to red and the long hairs protruding from them are light brown.

After feeding, the larvae drop to the ground to spin thin cocoons just beneath the soil surface where they will overwinter until the following spring.

Cultural management:

For fall webworm, removal of the web is the simplest management method. If the tent can be reached, it is possible to cut it out. The entire population of caterpillars will be inside. Tents that are out of reach near the tops of taller trees can be left alone since any defoliation they cause will not affect the overall health of an established tree.

Chemical management:

Chemical measures generally are not needed, since the late season feeding really does no long term harm to the tree. If insecticides are needed due to high populations of caterpillars, they should be applied as soon as the tents are evident since the insect is more susceptible when young.
The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.

For more information, contact The Morton Arboretum Plant Clinic (630-719-2424 or plantclinic@mortonarb.org).