Fall webworms unsightly, but not damaging

By Stephanie Hughes

OSU Extension master gardener volunteer

This time of year we begin seeing them – the webs of worms in trees, encompassing a group of leaves and causing damage (or at least concern) regarding tree health. While many patches of these webworms cause alarm, they don’t damage trees enough this time of year to cause significant concern.

Fall webworms begin as a snow white moth (some with black or brown spots), which mates, then lays masses of yellow eggs on the underside of leaves. The hairs from the female protect them, as these hairy caterpillars grow into dark, hairy, 2-inch caterpillars. Some have black heads, some have orange. These caterpillars begin weaving webs from branch ends, not in the crotch of a tree (not to be confused with tent caterpillars and/or bagworms).

Fall webworms (Hyphantria cunea Drury) are found throughout North America, feeding on more than 600 species of trees and shrubs, causing mostly cosmetic damage, and the unsightly webs. We most commonly see them on crabapple, hickory, cherry and walnut. At first, the young caterpillars eat the leaf surfaces, leaving the veins. Debris collects in the web. Mid-late summer dark-brown pupae crawl to mulch, leaf litter, or the soil to surround themselves with a transparent, hairy cocoon. Over winter, they mature and the moths emerge mid-March to April, mate, and the female lays up to 900 eggs. These eggs hatch in seven days, form a web and feed within, completing the cycle.

The caterpillars molt six times as they feed, adding to the debris in the web, which is enlarged to accommodate new leaves for a food source. It can spread up to 3 feet and envelop a small tree.

After feeding for four to five weeks, the caterpillars crawl down, spin cocoons, pupate in the mulch, leaf litter or soil. Sometimes in July or August, a new generation can develop, which may be another race. Eventually, the webs weather away.

Scouting trees during August will help observe damage before it becomes a problem in trees. The best way to rid your trees and shrubs of fall webworms is to pull the webs down, or tear the web to expose caterpillars to birds and wasps – natural predators. If you decide to spray, there are many organic options to try first. But it is too late in the season for most chemical controls. Remember to spray the fresh surrounding leaves. Spraying the webs is ineffective.