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Fall Webworm

August 15, 2006

The fall webworm, Hyphantria cunea, is noticeable throughout Illinois with the presence of small webs appearing on trees. This is the start of the second generation in the southern half of the state and the only generation in northern Illinois. This insect is typically quite evident in August and September, with silk webbing enclosing the ends of branches and associated foliage. Fall webworm caterpillars (= larvae) are pale green to yellow to almost whitish in color and generally possess black spots (two per each abdominal segment). They are covered with long white hairs.

They feed on a wide variety of trees, including birch, crabapple, hickory, maple, pecan, and walnut. Fall webworm larvae, unlike eastern tent caterpillar larvae, remain within the enclosed webbing and do not venture out to feed. Caterpillars consume leaves, resulting in bare branches with dirty webbing attached.

Although this feeding injury may ruin the aesthetic appeal of infested trees, it is not harmful to tree health as trees are primarily allocating resources for storage as opposed to producing new vegetative growth. The most appropriate control technique is simply to prune out the small webs that enclose the larvae. The use of insecticide sprays may be ineffective because the caterpillars remain in the webbing while feeding. If insecticides are applied, be sure to use high-volume spray that penetrates the protective webbing. The larvae are susceptible to natural enemies (parasitoids and predators), and you can assist them in reaching and attacking fall webworm larvae by opening up the webs.

Author: Raymond A. Cloyd


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