What causes the large bag- or tent-like webs on my pecan trees each year? Will they kill the tree?

Michael J. Hall, Bollich, Patricia A.  |  11/9/2006 11:05:34 PM

Fall webworm damage.

The webs that occur in pecan trees are formed by the caterpillar of an insect called the fall webworm. The adult fall webworm is a stout-bodied white moth with a wingspan of about one inch. The forewings are sometimes marked with brownish-colored or black spots. The caterpillars have orange heads and small bumps on the body from which long silky hairs arise. Full-grown caterpillars are 1 to 1¼ inches long. The fall webworm overwinters in the pupal stage in the leaf litter and surface layers of the soil. Adult moths are active in the summer and fall. After mating, the females fly into the trees and lay their egg masses on the undersides of the leaves. This is why it is not unusual to see webs high up in the tree canopy. The eggs hatch in about one week and the caterpillars that emerge begin feeding in groups. As the caterpillars grow, they construct large, loose, irregular-shaped silk webs, which cover the leaves, twigs and small branches. As the caterpillars continue to feed and develop, the webs are enlarged to enclose more leaves and twigs. Prior to pupation, caterpillars of the early generations leave the web and find protected areas, such as bark crevices, on the tree to pupate. Caterpillars of the last generation leave the web and drop to the ground, where they pupate in the leaf litter and soil. In yard trees, the large ugly webs seriously detract from the beauty of the tree but will not kill the tree. In commercial orchards, defoliation caused by feeding caterpillars can reduce the quality of the current season’s nuts, and if defoliation is extensive, reduce crop yields the following season. Massive defoliation over several years may weaken a tree, making it susceptible to other insects and diseases.

In properly managed orchards the fall webworm is seldom a pest. Insecticide sprays routinely applied to control other pecan insect pests also control webworms. In residential areas, fall webworm is difficult to control because homeowners rarely have the type of spray equipment required to adequately cover large pecan trees. When only a few webs occur on small trees, it is more economical to prune them from the trees. Periodic inspection will allow for removal of the webs before they become large, numerous and unsightly.

More detailed information on the distribution, description, life cycle, damage and control of the fall webworm can be found on the Fall Webworm fact sheet and on the Louisiana Recommendations for Control of Pecan Insects in Commercial Pecan Orchards guide on the Entomology page of the Pecan Station website.

Question answered by Dr. Mike Hall, Pecan Research-Extension Station entomologist.

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