Home Gardening

Fall webworms quite a nuisance
The Coast Gardener Newspaper and Web Column - September 15, 2001

What kind of worms are attacking my pecan tree?

This question has surfaced a lot recently. Fall webworms have been active this summer in trees such as pecan, persimmon, sweetgum and hickory.

This pest is a species native to the United States, and it occurs in most areas of the country. Host plant preferences vary from region to region. In the Western Appalachia and the Ohio valley regions, it seems to prefer American elm, maples or hickory. In the western United States, it is common on alder, willow, cottonwood, and fruit trees.

Two races or color phases of this insect are recognized. Larvae of the blackheaded group have black heads and tubercles (small bumps on the body), while the larvae of the redheaded group have red heads and tubercles. Both groups are covered with long silky hairs that arise from the tubercles. Mature larvae will measure 1 to 1 1/3 inches long. The adult moth is white and medium-sized with a stout body.

The insects overwinter in leaf litter and the surface layers of the soil. Emergence time is different for the two color phases. The females of the blackheaded group deposit eggs as a single mass in mid-March, while the females of the redheaded group deposit most eggs as double layers in mid-April. The eggs are deposited on the undersides of the leaves and hatch in about one week. As the larvae feed, they construct large silk webs, which cover leaves and small branches at the ends of limbs. The larvae move around inside the web and feed on the leaves. The webs grow larger and larger as the larvae mature. They then leave the webs and find protected places such as bark crevices or the soil surface. There are two to three generations per year. Data from the Mississippi State Entomological Museum show a difference in color patterns between the early and late emerging adults. The early emerging moths are white with black spots on the front wings and some black banding on the abdomen. Those solid white moths emerge beginning in mid-April.

Infestations do not occur in trees in thickly wooded areas but rather on those trees that occur on the borders of forested areas or in open lawn situations. Notice the trees along highway clearings or other open areas. They may be heavily infested, while a tree of the same species located several yards into the woods will not have a single web.

Fall webworms cause physical damage to trees by eating their leaves. Some people are concerned because webs covering trees detract from the beauty of them. Although treatment is possible, treatment is difficult. It is hard to spray a tree without the proper equipment that generates the pressures needed for coverage and web penetration. In fact, it may not be a good idea to spray, especially if the trees occur in residential areas. High pressure generates small droplets of insecticide that may drift into areas unsuitable for treatment. Pruning out reachable webs is possible; however, many webs may occur in the top portion of the tree well out of reach.

In most situations the only thing that can be done is to maintain good tree nutrition through proper fertilization and watering programs. Cleaning leaves and debris from around infested trees may help break the life cycle, too.

These archived gardening columns were written by Chance McDavid, former Harrison County Extension Agent.