Fall webworms enclose themselves and tree branches

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Last Updated: 
October 31, 2008
fall webworm

Fall webworm larvae enclosed in webbing. Photo by Steven Katovich, USDA Forest Service, Bugwood.org

fall webworm

Fall webworm larvae enclosed in webbing. Photo by Steven Katovich, USDA Forest Service, Bugwood.org

CORVALLIS, Ore. – Have you noticed cobwebby, silken caterpillar nests in deciduous trees in the late summer into fall? Most likely, these gauzy-looking tents at the end of tree branches are numbers of 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.

Feeding on more than 85 species of deciduous trees and shrubs in the United States, fall webworms are commonly seen in black walnut trees, willows, fruit trees and cottonwoods in Oregon, explained Jeff Miller, an entomologist in the Department of Rangeland Ecology and Management at Oregon State University.

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 OSU Extension nursery pest horticulturist Robin 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 1/2 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.

Considered to be a pest by many people, fall webworms are primarily a cosmetic nuisance, say Miller and Rosetta.

"They rarely cause significant damage, but are considered unsightly," said Rosetta, stationed at OSU's North Willamette Research and Extension Center in Aurora.

If fall webworm tents become bothersome, Miller advised that the best home remedy is to prune the infested branches off, "assuming they are within safe reach." Then make sure to destroy or remove the cut, tent-laden branches from the area, to avoid spreading the caterpillars elsewhere. 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 a measure, then you might try physically removing the tents, he said. But this is tricky, because if you merely squirt the tents with a jet of water or tear the tents up, this sometimes disperses the webworms to spread to other branches to create new tents there.

Miller 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.

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

Miller is the author of, "Caterpillars of the Pacific Northwest Forests and Woodlands," available online.

To see photos of fall webworms and the adult moth go to: http://oregonstate.edu/dept/nurspest/fall_webworm.htm
http://www.npwrc.usgs.gov/resource/insects/catnw/pht102.htm
http://www.npwrc.usgs.gov/resource/insects/macronw/9.htm

To learn more about the fall webworm, and to see pictures of these critters in action and their webs, visit Rosetta's OSU Pacific Northwest Nursery IPM website.

Author: Carol Savonen
Source: Jeff Miller, Robin Rosetta