Distribution & Hosts:

Native to North America, the fall webworm occurs throughout the United States and southern Canada. Its hosts include more than 100 species of deciduous forest, shade and fruit trees, with preferences varying from region to region.

Description:

Eggs are small, yellow or light green, and usually located in hair-covered masses on the underside of leaves. Mature larvae are 25-31 mm (1 - 1 1/4 inches) long and covered with silky hairs. Their color varies from pale yellow to green, with a black stripe on the back and a yellow stripe on each side. Head color varies from red to black. Pupation occurs in thin cocoons usually spun in the duff or just beneath the surface of the soil. The adult moth has a wingspan of 25-31 mm (1 - 1 1/4 inches) and is snowy white, usually with dark spots on the wings.

Life Cycle:

Adult moths lay their eggs from May to July. Eggs hatch within two weeks and the larvae immediately begin feeding and constructing webs. Larvae feed for four to eight weeks. The number of generations per year varies according to the region. The main generation occurs during July and August in the Northeast.

Damage:

Newly emerged larvae immediately begin to spin a silken web over foliage on the terminal portions of the branches. The larvae feed on the leaves within the webs. As the larvae grow, webs enlarge and enclose more foliage. Large portions of tree branches are commonly enclosed by such webs, and are most apparent from mid- to late-summer. Early stage larvae feed on the upper surfaces of the leaves, and late instar larvae eat entire leaves except for larger veins and midribs. Fall webworms are seldom numerous enough to cause total defoliation and severe damage to the tree. Even if they are present in large numbers, the damage is still not likely to be a serious threat to a tree because fall webworms are late season feeders and by the time the leaves are consumed the leaves have already performed most of their function to the plant. The insect is considered an ornamental pest due to the unsightliness of the webs; however, it is ordinarily of no great importance as a forest pest.

Control:

More than 50 species of insect parasitoids and 36 species of predators of the fall webworm are known in the U.S., yet they are not commercially available. 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, Bacillus thuringiensis 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 are first noticed (usually mid to late July).

Adapted from the Cornell Cooperative Extension and the Virginia Cooperative Extension, 1999