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.
On small trees, nests can be cut out and destroyed. Insecticides may be applied from mid- to late- summer or when webs are first found. Consult the Pest Management Guide for Horticultural and Forest Crops, Virginia Cooperative Extension Publication 456-017. More than 50 species of insect parasitoids and 36 species of predators of the fall webworm are known in the U.S., yet they are not commercially available.
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Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Alan L. Grant, Dean, College of Agriculture and Life Sciences; Edwin J. Jones, Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg; Jewel E. Hairston, Administrator, 1890 Extension Program, Virginia State, Petersburg.
May 1, 2009