Fall Webworms

Description

The fall webworm moth (Hyphantria cunea) is rather nondescript. It is mostly white with dark spots on the wings. The larvae are about 1 inch when full grown and pale yellow or green. There is a broad, dusky stripe running down the back, bordered on each side by a yellow stripe. These caterpillars are covered with tufts of long whitish hairs.

Biology and Habits

There are from two to four generations per year depending upon the locality in the state. The first occurs on mulberry as early as April in south Texas, and as late as June around Lubbock and Amarillo, attacking pecan, willow and poplar trees. Fall webworms over winter as pupae in silken cocoons under ground trash or on rough tree bark. Moths emerge to disperse and deposit eggs in hair covered masses on the underside of leaves. The newly hatched larvae begin feeding and spin a web over the foliage in which they feed. Sometimes, whole branches are covered with these webs. In extreme infestations, the entire tree may be covered with several webs. Larvae eat the tender parts of the leaf, leaving the larger veins and mid-rib. The last generation, which occurs in the fall, is usually the most damaging and gives this insect its name.

Severe defoliation may occur, weakening and sometimes killing trees. In fruit or nut orchards the insect usually is controlled with multiple applications of insecticides for other insect pests.

Control

Fall webworms may be controlled without using insecticides by thoroughly inspecting trees that have a history of webworm infestations. In small trees where the upper parts are easily reached, egg masses on leaves may be located and destroyed. In larger trees webs can be pruned out and destroyed.

Many parasites and predators attack fall webworms. Paper wasps that search the webs and carry webworms to their nests to feed the developing wasps are an important natural control. Nests of paper wasps that pose no threat to humans should be protected.

If webs are too numerous or out of reach for pruning, insecticides may be needed to prevent damage. A high pressure sprayer is necessary to reach the top of tall trees and to penetrate the sometimes dense webs. Breaking the webs with a long pole helps the insecticide applications to contact the larvae. The most efficient control is achieved when applications are made after eggs hatch and before larvae develop large, dense webs. Use products containing acephate (Orthene*), Bacillus thuringiensis (Dipel, Thuricideg, Bacturs)carbaryl (Sevin), diazinon, malathion, chlorpyrifos (Dursban*), methoxychlor or trichlorofon (Dylox). Since complete coverage is needed for good control, a commercial tree spraying service may be necessary for large trees or heavy infestations.

Insecticide label clearances are subject to change and changes may have occurred since this publication was printed. The pesticide USER is always responsible for the effects of pesticides on his own plants or household goods as well as problems caused by drift from his property to other properties or plants. Always read and follow carefully the instructions on the container label.