Key Features
- Webs covering leaves and branches
- Chewed leaves
- White hairy caterpillars
Symptoms
Large webs are found at the edge of the canopy. Heavily infested trees can be completely covered by the webs produced by caterpillars of the fall generation. Caterpillars are yellowish-green with black spots and long white hairs, and grow up to 1.5". Young caterpillars skeletonize leaves and older caterpillars consume them. Caterpillar feces falling from trees can be a problem during heavy infestations.
Biology
Brown pupae spend the winter in leaf litter in a loosely webbed cocoon. In June, adults fly and lay large white egg masses (>200 eggs per mass) under leaves. Eggs hatch into caterpillars that feed for 4-6 weeks. A second generation occurs in the fall.
Management Recommendations
Control of fall webworm caterpillars is best achieved if actions are taken before the tree is covered with webs from either the first, or the second generation. Small webs can be simply pulled off and destroyed if easy to reach, and only a small proportion of the tree is affected. Pesticides should be used when there are too many webs to make web removal practical. To kill caterpillars without creating other pest problems (spider mites, scale insects) it is best to use biorational pesticides (spinosad, Bacillus thuringensis, chlorantraniliprole). Caterpillars may have to feed on treated leaves for 1-2 days to get a lethal dose of these materials. When pests are discovered late in season and extensive damage is present, it may be necessary to spray a rescue material to kill caterpillars quickly. Rescue treatments include Sevin(carbaryl) or a pyrethroid (eg. bifenthrin, cyfluthrin, deltamethrin, lambda-cyahalothrin, permethrin). To protect bees avoid applying insecticides when trees are flowering. Pesticides alone are not enough to remove the webs from the trees. Trees with heavy webs can remain unsightly well into the winter. Webs will be removed if pesticides are applied with a high pressure sprayer by a professional applicator. Homeowners seeking to remove webs after caterpillars have been killed could use a strong stream of water from a garden hose that is fitted with a spray nozzle.
Effective Pesticides
Active Ingredients include: Bacillus thuringiensis var. Kurstaki, Bifenthrin, Carbaryl, Chlorantraniliprole, Cyfluthrin, Deltamethrin, Indoxacarb, Permethrin, Pyrethrin, Spinosad
