The second half of summer and fall is the season of fall webworm. Fall webworm attacks many hosts, over 85 known species of deciduous trees, including elm, hickory, pecan, plum, chokecherry, poplar, walnut and willow. In fact, almost all fruit, shade and ornamental trees, except conifers, can be affected by fall webworm. A similar insect, Mimosa webworm, affects honeylocust. Webworms should not be confused with bagworms, which are the larvae of moths and build small, one inch bags or "cocoons" in spruce and other trees.
Adults of this native insect are white moths, with reddish-orange front legs and a 1¼-inch wingspan. Immature insects are pale yellowish caterpillars with red heads and reddish-brown spots. An alternate color variation among the larva is yellow-green caterpillars with black heads a broad dark stripe on the back and black spots. Adult moths emerge in late spring or early summer and lay eggs in masses on the undersides of leaves. The larvae emerge 10-14 days later and begin feeding in groups within a small webbed mass of leaves at the ends of branches. The caterpillars have many long, fine hairs on their backs. There are one to two generations per year in Nebraska.
Homeowners often spot the large, silken web masses of fall webworm in the upper branches of their trees in late summer. The webbing provides protection from some predators and the caterpillars feed inside the web until all leaves are devoured, then additional leaves are encased in the web. Webbed areas of leaves grow larger as the caterpillars mature, becoming a messy, ugly eyesore as it is filled with shed skins, excrement and leaf fragments.
The first generation of caterpillars matures in about six weeks. Then they drop to the ground and enter the soil, where they pupate into adults and re-emerge to lay eggs for the second generation. Some larvae may pupae under loose bark, in leaf litter beneath the tree, or within the webbing. Caterpillars of the second generation hatch and feed from approximately early August through late September. Then once again, the mature caterpillars drop to the ground and enter the soil to overwinter. Because the insects overwinter beneath host plants, trees that have been attacked in the past will very likely have insects the following year, too.
Although unsightly, feeding by fall webworms is rarely seriously damaging to large trees; however, several years of defoliation for small ornamental trees can weaken them. Removal of nests in early summer when only a few leaves are involved is the best method of control. Prune out the branches affected and crush or burn them. Biological insecticides such as Bacillus thurengiensis, B.T., are also effective. Thoroughly cover leaves next to the nest, and as the larvae ingest the insecticide they will be killed.
Sarah Browning is a Horticulture Extension Educator with the University of Nebraska- Lincoln Extension in Dodge and Saunders Counties. She can be contacted by phone at 727-2775: by mail at 1206 W. 23rd Street, Fremont, NE 68025: or by e-mail at sbrowning2@unl.edu
