Two of the most common pests of deciduous trees are the Eastern tent caterpillar and the fall webworm, whose conspicuous gauzy tents deface roadside trees, home landscape trees, and fruit tree orchards. Although people often confuse the two or mistake them for the gypsy moth, which does not spin a web at all, their life cycles, feeding habits, and time of appearance differ significantly.
A fully grown Eastern tent caterpillar can reach 2 1/2 inches, with a deep black, hairy appearance, and showy white stripes running lengthwise down its back bordered by pale blue spots. They spin white silken threads around a one-inch cocoon that encloses a dark brown pupa. Later, adults emerge as dark chocolate-brown moths, with two white stripes running obliquely across the forewings.
The Eastern tent caterpillar overwinters in an egg mass, which has been deposited around a small twig. Each mass is about 1/2-inch long, oval-shaped, and irridescent brown. It contains as many as 300 eggs. As the leaves on the host tree appear in early spring, the eggs hatch and the newly emerged larvae begin spinning protective silken tents in the twig crotches. Colonies of two to three hundred caterpillars rest in these tents at night or on cloudy, rainy days and emerge on sunny days to feed on tender young leaves. By the end of May, the caterpillars leave the tree to form cocoons, remaining in the pupa stage for approximately two weeks. By late June, adults will emerge to mate and lay their eggs on twigs of the host plant. Producing only one generation per year, these eggs will not hatch until the following spring.
Trees in the rose family, such as crabapple, apple, and cherry, are the primary hosts of the Eastern tent caterpillar, but numerous fruit trees and deciduous ornamental trees can be attacked. The damage is done in the larval stage. Caterpillars will begin building a tent or web in the fork of a tree, but as they leave the tent to feed on new leaves, a silken strand follows them and enlarges the web as the caterpillar eats. Depending on the size of the tent population, an infestation of Eastern tent caterpillar is capable of totally defoliating a tree. Usually new leaves will emerge later in the season, but total defoliation can weaken a tree and make it susceptible to other insects and diseases.
Remove and destroy the tents when they are first noticed. It is best to remove at night or on cloudy days when caterpillars are inside the nest and not out feeding.
If needed, chemicals should be applied as soon as the tents are evident since the insect is more susceptible when young. Bacillus thuringiensis var. kurstaki is a low toxicity product that can be sprayed on young larvae (mature larvae will not be killed by Btk).
The fall webworm differs from the Eastern tent caterpillar by the time of year in which it is seen, its feeding habits, and the placement of its protective tent.
In its larval state, the webworm is a 1-inch caterpillar, usually pale yellowish-green with a broad, dusky stripe down its back and a yellow stripe on each side. They are covered with long, silky gray hairs that arise in tufts from orange-yellow or black tubercles. The color of the head can be red to black. As an adult, the fall webworm emerges as a white moth with a wingspan of about 11/2 inches. Occasionally there are a few black or orange markings on the body and legs.
Adult fall webworms appear mostly from May to August and deposit egg masses of up to 1500 eggs on the lower surface of leaves of a host tree. As they hatch, larvae quickly begin spinning their webs over the leaves on which they feed. This web enlarges to cover more foliage as the larvae continue to feed. If a tree is heavily infested, it is possible to have several branches enclosed in webs. After feeding, the larvae drop to the ground to spin thin cocoons just beneath the soil surface where they will overwinter until the following spring, emerging as adult moths. Approximately 120 species of deciduous trees are host to the fall webworm, with mulberry, maple, crabapples, birch, chokecherry, walnut, and willow being most susceptible. The damage occurs in late July and August as the larvae feed on leaves while inside their tents. Immature larvae eat only the outer surfaces of the leaf and leave the leaf veins untouched, while mature larvae will consume the entire leaf right down to the petiole. Nevertheless, given the time of year that the feeding takes place, the fall webworm’s damage is more of a cosmetic problem to the tree than any serious health threat. If the fall webworm has infested a small tree or a recent transplant, some control measures may be warranted.
As with the Eastern tent caterpillar, removal of the web is the most reasonable method of control. If the tent can be reached, it is possible to either cut off the web or open it up and physically remove the caterpillars and destroy them by crushing or immersing them in an alcohol solution. Tents that are out of reach near the tops of taller trees can be left alone since any defoliation they cause will not affect the overall health of an established tree.
Chemical measures generally are not needed, since the late season feeding really does no long term harm the to tree. If needed, chemicals should be applied as soon as the tents are evident since the insect is more susceptible when young.
Contact the Plant Clinic (630-719-2424 or firstname.lastname@example.org) for current recommendations. Use pesticides safely and wisely; read and follow label directions.The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.