Linda Haugen, USDA Forest Service, Bugwood.org

Fall Webworm (Hyphantria cunea)

Last year there was a large infestation of ‘Fall webworm’ in the Comox Valley and across many parts of Vancouver Island. You may have noticed the large ‘tent’ webs on red-alder trees along the roads and highways starting in July and August, and perhaps in your gardens too.

While the caterpillars and their webs may be unsightly during the summer and fall, they do not cause any harm to the trees they infest.

Life Cycle

The white adult moths emerge from overwintering cocoons in the summer and lay their eggs on the underside of leaves. The moths prefer red-alder, wild cherry and willow, but other native and landscape trees can be affected too. The eggs quickly hatch into pale yellow, inch-long caterpillars, with white hairs, a black head and dark stripes along their backs. The Fall webworm caterpillars mainly feed on leaves near the tips of branches, where they build large webs for protection. Eventually the caterpillars leave the host tree and build cocoons nearby to overwinter.

James B. Hanson, USDA Forest Service, Bugwood.org

Treatment

Where webs are accessible, they can be pruned away or pulled apart, to remove or damage a colony. However, this can be hard to do and is not always effective, and you are often still left with an empty web swaying in the wind for several months to come, before the tent eventually falls apart.

Grow Tree Care can help prevent the unsightly Fall webworm tents from forming by spraying susceptible trees with BTK when the caterpillars have just hatched from their eggs.

BTK is a naturally occurring bacteria present in the soil that kills caterpillars by disrupting their digestive system, but is harmless to other insects, plants and animals.

Timing of the spray application is important for effective control as, too early and the caterpillars are not yet present, and too late and the webs have already formed. For this reason, three applications of BTK are recommended, starting in mid June and spaced ten days apart. This will help to catch the Fall webworm in the desired part of its life cycle and prevent webs from forming in your trees.

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Main Image: Linda Haugen, USDA Forest Service, Bugwood.org