Bug of the Month

Fall webworm tents don’t really hurt trees

You may have noticed those cobwebby caterpillar nests in deciduous trees this fall and summer. Most likely, these gauzy-looking tents at the end of tree branches are the work of fall webworm caterpillars (Hyphantria cunea).

Fall webworms are known to infest more than 100 species of deciduous trees and shrubs, but are most commonly seen in black walnuts, willows, orchard trees and cottonwood, according to Lynn Royce, entomologist with the Oregon State University Extension Service. They spin a silk “tent” near the end of branches of leafy trees and shrubs and feed on the leaves they have enclosed.

Webworm caterpillars are yellowish brown with black and orange bumps and have long tufts of whitish hairs. They grow to about 1.5 inches long, then overwinter as pupae in a dark brown cocoon in protected places, such as in bark crevices or on the ground in litter or duff. In the spring, they emerge as large silky white tiger moths with black spots. These adults lay yellow or white eggs in the early summer on the undersides of leaves.

According to Royce, these webworm caterpillars are more of a cosmetic nuisance than anything. They cause little damage because they usually eat only a small portion of the leaves off any given tree. They consume the leaves so late in the growing season that they would be shed soon anyway.


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