The fall webworm, a common pest of many shade and ornamental trees in Texas, is native to North America and occurs throughout the United States and southern Canada. Its hosts include more than 100 species of broadleaf, trees including pecan, persimmon, black walnut sweetgum, elms, hickory, maple and cherry. This insect pest mainly affects the appearance of the host tree and is more a nuisance than a threat to the tree’s health. The larvae, which cause the damage, are of two types - the blackheaded form and the redheaded form.
The food habits, biology, and markings on the larvae and adults are different for the two forms. In Texas, adults of the blackheaded form generally appear about one month earlier than the redheaded form.
Young larvae of the black ’ headed form are yellowish green to pale yellow with two rows of dark bumps along the back. The head is black and covered with fine hair. The mature larvae of the blackheaded form is yellowish or greenish with a broad dark stripe along its back. The redheaded variety is tawny or yellowish tan with orange to reddish bumps. The larvae of the blackheaded form construct a flimsy web; that of the redheaded form is larger and more compact. Full-grown larvae of both varieties are about 1 inch (25mm) long.
The eggs hatch about two weeks after they are laid and the young larvae immediately begin to spin a silken web over the foliage on which they feed. As they grow they enlarge the web to cover more and more foliage. On large trees, complete branches may be covered, while on smaller trees, the entire plant may be encased in webbing. Young larvae skeletonize the upper leaf surface while older larvae devour the entire leaf except for the large veins and midrib. Larvae usually require 4-8 weeks to develop. As they approach maturity, the larvae leave the webs and wander and feed as they search for suitable pupation sites. Pupation generally occurs in thin silken cocoons spun in the duff or just beneath the surface of the soil.
The adult moths of the first generation usually appear by May in Texas. They have a wingspread of about 1 1/2 inch (30-42mm) and are white in color with dark spots on the wings. The coloration of the adults, as well as the larvae, can be quite variable. Shortly after the moths emerge, they mate and the female lays several hundred eggs in a mass on the underside of the leaves. She covers them with hairlike scales from her body so they appear as a white, cottony patch on the leaf. There may be as many as four generations per year in southern areas of the state.
Since both the fall webworm and the tent caterpillar construct webs in the crowns of their hosts, it is important to distinguish between the two. The web of the fall webworm is more flimsy and encloses the ends of the leaves of individual branches. The tent caterpillar confines its web to limb crotches and flat mats along branches, and they are only found early in the spring.
The fall webworm has a large complex of natural enemies - more than 50 species of parasites and 30 species of predators are known in America. These beneficial insects along with disease, starvation and unfavorable weather conditions usually keep webworm populations at tolerable levels. Should direct control become necessary, nests of the webworm may be pruned from high value trees and destroyed. Chemical controls recommended for the fall webworm include Sevin and the bacteria Bacillus thuringiensis. All suggested chemicals must be currently registered and labeled for use by the US Environmental Protection Agency and the Texas Department of Agriculture. Before using any pesticide, read and carefully follow all application directions, cautionary statements and other information appearing on the label.