The fall webworm, a common native caterpillar, occurs throughout the United States and has caught my eye in several areas of Columbia County recently. As the name implies, it’s a web-maker. The larvae feed in colonies on foliage of hardwood trees and spin grayish silk webs around leaves in the process. Typically, webs are located at the outer ends of branches.
However, when webworm populations are high, webbing is more extensive and may cover the entire crown of the tree. Webworms are capable of defoliating and causing damage to forest trees, but it is more a pest of shade and ornamental trees in the landscape.
The fall webworm has one of the widest host ranges of any caterpillar. Fall webworms feed on over 85 species of trees in the US, according to UGA Extension horticulturist Bob Westerfield. Hickory, American elm, pecan, walnut, and various fruit trees are their preferred food species. Willow, persimmon, maple, sweetgum, cottonwood, and alder are also host trees. Pecan trees are infested more frequently than others.
Fall webworms can have up to four generations in a year with most of the activity occurring in late summer and early fall. The larvae or caterpillars are hairy with distinct paired dark spots on each segment of the back.
They can be variable in color, but the primary color for this area is a light yellow to pale green with white hairs. The mature caterpillar, about one inch long, will twitch and jump when disturbed to deter potential predators. The caterpillars transform into a white moth with a wingspan of one-and-a-half inches.
The fall webworm will winter as a pupa in a flimsy light-colored cocoon.
These cocoons can be found in protected areas such as bark furrows, crevices along the sides of buildings, and under leaves and trash on the ground. The adult moths emerge in June to July and begin laying eggs on the underside of leaves. Females lay their eggs in groups of 300 to over 1000 on foliage of host trees. When the eggs hatch, the young larvae feed together.
They will skeletonize the leaf and start forming their web. The larvae feed inside the tent-like web. The young larvae do all of their feeding inside of the tent. When full grown, the caterpillars leave the tent to find a protected area for the winter.
Controlling fall webworms is fairly easy. The best way to control them is to remove the web from the tree as they form. Simply use a pole pruner to reach them and remove the portion of the limb that has the web.
Since fall webworms don’t feed outside of the web, this will remove all of the caterpillars. Biological control can also be encouraged for fall webworm control. Over 80 species of predators have been documented for fall webworm.
Wasps, birds, predatory stink bugs, and parasitic flies are the most important. These predators should be in good supply if you aren’t regularly applying insecticides in your landscape. Another biological control method is BT (Bacillus thuringiensis), a natural bacterial insecticide.
BT is effective against fall webworms when applied to small caterpillars. Thoroughly cover leaves adjoining a nest. As the leaves are incorporated into the nest, the BT will be ingested and kill the caterpillars.
There are insecticides that you can use, but they can be hard to apply since most of the webs are located in the tops of trees.
Typically, fall webworms are only considered unsightly and do not typically cause major damage to the trees they infest. However in some circumstances, the host trees can be completely defoliated. Healthy hardwoods usually survive and recover without permanent injury. Several consecutive defoliations can cause dieback in the crown and may contribute to the death of these trees.