Worms That Make Webs in Trees

Several different kinds of moths lay eggs in trees. The eggs hatch into caterpillars that create unattractive webs and feed on the leaves of deciduous-tree species. These insect pests rarely harm healthy or established trees, but they can further weaken young or stressed trees and may cause death. Homeowners use biological, chemical and mechanical controls to reduce caterpillar populations.

Tent caterpillars (Malacosoma species) are the larvae of light brown, night-flying moths. Their larvae vary in color depending on the species. Eastern tent caterpillars have blue-spotted, hairy black bodies; yellow, brown and white lines run down their backs and sides. Western tent caterpillars are orange and black, while forest tent caterpillars have blue bodies with black and white markings. These pests form colonies in willow, cottonwood, peach, hawthorn, cherry and plum trees, among many other species.

Fall webworm caterpillars (Hyphantria cunea Drury) feed on species such as crabapple, walnut and hickory. Young webworms have light yellow bodies with black markings. They darken in color as they mature; mature webworms are about 1 1/2 inches long, with hairy greenish bodies and yellow stripes that run down their sides. Some have reddish-orange heads, while others have black heads depending on the variety. The adults are usually white, but some have black spots. They pupate in cocoons sheltered inside 1/2-inch-long brown, cylindrical-shaped pupa.

Fall webworms create tent-like, silken nests at the ends of the branches, enclosing areas of the foliage inside the web. The young caterpillars feed on the soft tissue between leaf veins, but older fall webworms devour entire leaves. Tent caterpillars build silken nests in the crotches between branches and congregate in large colonies inside the tent during warm weather. They feed on leaves in the evenings and mornings when the weather is cool. Heavy infestations of tentworms or fall webworms can defoliate entire trees.

You can control fall webworm and tent caterpillar populations by pulling down their webs, which leaves them vulnerable to attacks from predatory insects and birds. You can also remove and dispose of tent caterpillar egg masses. Tachinid flies kill tent caterpillars by laying their eggs on the caterpillars. When the eggs hatch, the maggot-like larvae feed on the caterpillars. Spray hard-to-reach nests with Bacillus thuringiensis, a biological insecticide that sickens and kills certain kinds of caterpillars. Traditional insecticides also help control caterpillar infestations.