Fall webworm appearing

I just saw a problem on a tree on my place this week that will generate a lot of questions in the next month or so. There was the start of a web that covered all the leaves on the last 6 to 12 inches of the branch. This is caused by the larvae of an insect called the fall webworm.

Late summer and fall is the season for fall webworm. Fall webworm attacks many species of deciduous trees… in fact, almost all fruit, shade and ornamental trees, except conifers, can be affected by fall webworm.

Adults of this insect are white moths. 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 tips of branches.

The webbing provides protection from 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. Homeowners often spot fall webworm as they enlarge their silken webs in late summer.

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 pupate 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 are likely to have these insects the following year, too.

Although unsightly, fall webworm feeding rarely damages large, well-established trees. However, several years of defoliation on newly transplanted or small ornamental trees can weaken them. Remove the nests in early summer when only a few leaves are involved is the best method of control.

Break up the webbing with a rake or a strong stream of water. Many of the caterpillars will be knocked out of the web onto the ground, and will be killed by birds or predatory insects. If it won’t affect the appearance of the tree, you can also clip off the end of the branch and destroy the larvae living in the webbing.

If you are going to treat with an insecticide, biological products containingBacillus thurengiensis or B.T. are effective. Other insecticides, such as permethrin and bifenthrin, will also provide good control. Thoroughly cover leaves next to the nest, and as the larvae ingest the insecticide they will be killed.

For more information on fall webworm control, contact your local UNL Extension office.

John Wilson