Fall Webworm

27 July 2005

Virginia Cooperative Extension, Charlottesville/Albemarle County Office
460 Stagecoach Road, Charlottesville, Virginia 22902
phone: 434.872.4580   fax: 434.872.4578

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If you have noticed webbing on some of your ornamental trees lately you might think we had a relapse of the tent caterpillars from earlier this spring. Actually what you are probably seeing is the webbing of the fall webworm. The fall webworm, Hyphantria cunea (Drury), is most often discovered when the unsightly, light gray, silken webs on the trees in summer and early fall are observed. Webworms enclose leaves and small branches in their nests, unlike the tent caterpillars which make a smaller nest in the crotch of branches.

Native to North America, the fall webworm occurs throughout the United States and southern Canada. Its hosts include more than 100 species of deciduous forest, shade, and fruit trees, with preferences varying from region to region. Newly emerged larvae immediately begin to spin a silken web over foliage on the terminal portions of the branches. The larvae feed on the leaves within the webs. As the larvae grow, webs enlarge and enclose more foliage. Large portions of tree branches are commonly enclosed by such webs, and are most apparent from mid- to late-summer. Early stage larvae feed on the upper surfaces of the leaves, and late instar larvae eat entire leaves except for larger veins and midribs. The insect is considered an ornamental pest due to the unsightliness of the webs; however, it is ordinarily of no great importance as a forest pest.

Eggs are small, yellow or light green, and usually located in hair-covered masses on the underside of leaves. Mature larvae are 25-31 mm long and covered with silky hairs. Their 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. Pupation occurs in thin cocoons usually spun in the duff or just beneath the surface of the soil. The adult moth has a wingspan of 25-31 mm and is snowy white, usually with dark spots on the wings.

From May to July, adult moths lay their eggs. Eggs hatch within two weeks and the larvae immediately begin feeding and constructing webs. Larvae feed and webs continue to enlarge for four to eight weeks. There are at least two generations per year in the South.

More than 50 species of insect parasitoids and 36 species of predators of the fall webworm are known in the U.S. so there is free help available out there. On small trees, nests can be cut out and destroyed. Soap or other insecticides may be applied from mid- to late- summer. If you choose to use insecticides they are most effective when applied when larvae first begin to feed in early summer but this will most likely harm the beneficial parasites and predators as well. Try to withhold contact insecticide sprays until it is certain that predators and parasites are not present in sufficient numbers to control the webworms. Applications may need to be repeated in July if populations persist. Bacillus thuringiensis (B.t.) is the least toxic insecticide for non-target organisms as it only affects certain orders of insects. Use formulations with UV protectants and thoroughly cover leaves next to nests. As these leaves are incorporated into the nest and eaten, the B.t. will be ingested.

Other chemicals currently recommended include acephate, bifenthrin, carbaryl, dycarb, esfenvalerate, and permethrin. No matter which pesticide you use it is important to read the label before you buy and/or apply to be safe.

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This fall Virginia Cooperative Extension will be offering Master Gardener training in Greene County. For more information, call (434) 872-4580.

For more information about landscape topics contact your local Virginia Cooperative Extension office. Many county Extension offices have a help desk that is staffed by Master Gardener volunteers. These volunteers are trained to answer questions about home and landscape pest problems. The local Virginia Cooperative Extension office numbers are Albemarle 872-4580, Fluvanna 591-1950, Greene 985-5236, Louisa 540-967-3422, and Nelson 263-4035.