Fall webworms have begun to appear in tree branches

Fall webworm
Fall webworm larvae enclosed in webbing. (Photo by Steven Katovich, USDA Forest Service, Bugwood.org)
Last Updated: 
August 12, 2011

CORVALLIS, Ore. – Silken caterpillar nests that look like cobwebs have begun to appear in deciduous trees. The gauzy-looking tents at the end of tree branches are most likely fall webworm caterpillars working as a group to make a home.

These caterpillars are the larvae of a native species of tiger moth known to entomologists as Hyphantria cunea. They are considered to be a pest by many people, but are primarily a cosmetic nuisance, according to Oregon State University entomologists Jeff Miller and Robin Rosetta.

The caterpillars feed on more than 85 species of deciduous trees and shrubs in the United States and are commonly seen in black walnut trees, willows, fruit trees and cottonwoods in Oregon.

Fall webworms can be distinguished from tent caterpillars by how they enclose themselves in their web along with their food, the end of a leafy tree branch, explained Rosetta. Tent caterpillars are found outside their much smaller web, she said.

Webworm caterpillars, related to wooly bear caterpillars, are black with yellow to golden-orange bumps under a cloak of long tufted white hairs. They grow to about 1.5 inches long, and then overwinter as pupae in a brown cocoon in protected places, such as in bark crevices or on the ground in litter or duff.

The adult, a large silky white tiger moth with black spots on its underside, flies in mid-summer. After mating, the female moth will lay hundreds of yellow or white eggs on the undersides of leaves.

They rarely cause significant damage, but are considered unsightly, Rosetta said. If fall webworm tents become bothersome, Miller advises that the best home remedy is to prune off the infested branches. "Then make sure to destroy or remove the cut, tent-laden branches from the area, to avoid spreading the caterpillars elsewhere," he said. Sometimes burning is a practical way to get rid of infested branches, but it is not always safe, or allowed in all areas.

If pruning branches seems too drastic try physically removing the tents, Rosetta said. But if you merely squirt the tents with a jet of water or tear the tents the webworms can spread to other branches to create new tents there.

She suggests submerging the plucked tents and worms in soapy water. "And remember to pay attention after removing tents to make sure more new tents are not being made by recently dispersed webworms," she said.

Next year, be alert for new webworm tents. A new infestation might occur next year, regardless of your actions this year.

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Author: Judy Scott
Source: Jeff Miller, Robin Rosetta