WSU Extension

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Caption: Juniper webworm damage
Photo by: A.L. Antonelli
  
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Arborvitae : Juniper webworm
(revision date: 4/22/2015)


Biology
The adult juniper webworm is a copper-brown moth with white bands on the edges of the front wings. The moth is about 1/2" across. The webworm caterpillars initially feed by mining inside leaves. As they mature, they gather to feed in small colonies or nests of webbed foliage. The dark-headed caterpillars are yellowish to brown with dark brown lines on the back. They overwinter in the nest and resume feeding in the spring. Fully mature caterpillars are about 1/2" long. Damaged foliage turns brown and is covered with dirty webbing. Due to the caterpillars' habit of feeding deep in the plant canopy, damage may be easily overlooked.
Management Options

Non-Chemical Management
  • Hand-pick and destroy caterpillars.
  • Prune out and destroy nests.
  • Several natural enemies help control juniper webworm populations. Avoid use of broad-spectrum insecticides which may kill these beneficial predators and parasites.
Select non-chemical management options as your first choice!

Chemical Management


Begin monitoring in early spring (March) and apply when caterpillars are present. Apply Bt only when caterpillars are feeding. Use a spreader-sticker with liquid Bt formulations. Homeowners should not make foliar applications to trees over 10 ft tall. Consult a commercial pesticide applicator for treatment of trees and shrubs over 10 ft. tall.

Listed below are examples of pesticides that are legal in Washington. Always read and follow all label directions.
  • ferti-lome Dipel Dust
    Active ingredient: Bacillus thuringiensis subsp. kurstaki  |  EPA reg no: 7401-290
  • Safer Brand Caterpillar Killer/Trees, Shrubs, & Vegetables Conc
    Active ingredient: Bacillus thuringiensis subsp. kurstaki  |  EPA reg no: 42697-23
  • This list may not include all products registered for this use.
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Caption: Juniper webworm damage
Photo by: A.L. Antonelli
Caption: Juniper webworm larva
Photo by: K. Grey