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Fall Webworm

by Don Janssen, UNL Extension Educator

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The fall webworm is most often discovered when the unsightly, light gray, silken webs are observed on the trees in late summer and early fall. Webworms enclose leaves and small branches in their nests, unlike the tent caterpillars which make a smaller nest in the crotch of branches. The large silk webs enclosing tips of branches are sure signs of fall webworms. The caterpillars remain inside the webbing, and if food runs out new foliage is encased.

The larvae color varies from pale yellow to green, with a black stripe on the back and a yellow stripe on each side. Head color varies from red to black. The caterpillars make distinct jerking movements in unison if the nest is disturbed. The adult moths are about one inch long and range from pure white to white with a few black spots. This pest overwinters in the pupal stage. Pupae are usually in the ground but can be located in old nest remains, under loose bark and in leaf litter.

Though the webs are very unsightly, damage to most trees is considered to be insignificant. This pest tends to go through periodic population explosions. Outbreaks every four to seven years may last for two to three years and then natural control agents greatly reduce the activity.

Small nests can be pulled out of small to medium trees. Monitor trees early to detect the nests when only several leaves are involved. These small nests can be easily crushed. Do not burn or torch the nests in trees as this may do additional damage to the tree.

Over 80 species of parasites and predators have been identified in North America. Social wasps (yellow jackets and paper nest wasps), birds, predatory stink bugs and parasitic flies and wasps are the most important.

The bacterial insecticide, Bt, is quite effective against fall webworms if it is applied when the larvae are small. Use formulations with UV protectants and thoroughly cover leaves next to nests. As these leaves are incorporated into the nest and eaten, the Bt will be ingested.

Extensive nests may occur in tall trees which are difficult to spray with ground equipment. These trees can often be treated with translocated systemics applied to the soil for root uptake or injected.

(This resource was updated August 2007 and appeared in the Lincoln Journal Star Newspaper Sunday edition. For information on reproducing this article or using any photographs or graphics, read the Terms of Use statement)

University of Nebraska-Lincoln Extension in Lancaster County is your on-line yard and garden educational resource. The information on this Web site is valid for residents of southeastern Nebraska. It may or may not apply in your area. If you live outside southeastern Nebraska, visit your local Extension office

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University of Nebraska-Lincoln
in Lancaster County
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Lincoln, NE 68528
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