Fall Webworm

The fall webworm is a widely distributed native pest of shade trees and shrubs and appears from late summer through early fall. It feeds on almost 90 species of deciduous trees commonly attacking hickory, walnut, birch, cherry, and crabapple.

This species acts similarly to the eastern tent caterpillar, but the fall webworm constructs its nest over the end of the branch rather than at tree crotches. The large conspicuous webs contain caterpillars, dead partially eaten leaves, and fecal droppings.

White hair-covered egg masses contain several hundred light yellow eggs. Young larvae are pale yellow with 2 rows of black marks along their bodies. When fully grown, they are covered with whitish hairs that originate from black and orange warts. Larvae vary as to their coloring and markings, but are usually greenish with a broad, dusky stripe along the back with a yellow stripe along the side. The pupal stage is brown. Adult coloration varies considerably from pure white to white with black spots with a wingspread of about 32 mm.

Life cycle:The large silk webs enclosing tips of branches are sure signs of fall webworms. The caterpillars remain inside the webbing, and if food runs out new foliage is encased. The caterpillars are covered with long white to yellowish tan hairs. Two races of fall webworms occur in North America, the blackheaded and redheaded races. The blackheaded race has caterpillars which are light greenish-yellow to pale yellow with two rows of distinct black tubercles. The redheaded race is more tan in color with orange to reddish tubercles. Both races are found in Ohio. The caterpillars make distinct jerking movements in unison if the nest is disturbed. The adults are about one inch long and range from pure white to white with a few black spots.

This pest overwinters in the pupal stage. Pupae are usually in the ground but can be located in old nest remains, under loose bark and in leaf litter. The adults emerge from late May into July. The eggs are usually deposited in a single (blackheaded race) or double (redheaded race) layer of several hundred eggs on the undersurface of leaves. The mass is lightly covered with scales from the female’s abdomen. The eggs hatch in about a week and the small mass of caterpillars web over single leaves and feed by skeletonizing. As the caterpillars grow, they web over additional leaves and finally are able to eat the entire leaf. The larvae mature in about six weeks, at which time they drop to the ground to pupate. The moths emerge over an extended period in two generations can normally be completed in Ohio. In southern states, adults can emerge in mid-March and up to four generations can be completed.

Damage:The larval stage of this pest skeletonizes and consumes leaves inside the protection of a tent-like web that they enlarge as they require additional food and grow. They may defoliate a tree occasionally, but rarely kill it. On shade trees webs usually occur on occasional branches. They may not injure the tree appreciably, but they reduce its ornamental value.Fall webworm larvae have been known to feed on over 85 species of trees in the United States. Pecan, walnut, American elm, hickory, fruit trees and some maples are preferred hosts in the most of Ohio. Persimmon and sweetgum are also readily attacked in southern Ohio while willow, cottonwood, and alder is only occasionally attacked.This pest usually eats leaves late in the season and the nests are generally concentrated to limited areas. Because of this, little real damage is done to most trees. However, the nests can look very unsightly and multiple generations in long summers can lead to significant defoliation.

Control:Various species of natural enemies help to manage this native insect. Birds and many insect predators and parasitoids attack the larval stage. Eggs may also be destroyed by predators and insect parasitoids. It is possible to reduce this pest’s population by mechanical control. When the webbed branches are within reach, they can be pruned and destroyed. This may be practical if the webs have not become too large and the aesthetic shape of the woody ornamental plant is not reduced by pruning.Apply a registered insecticide when webs and larvae are small. This usually occurs sometime during July. The entire infested plant does not need to be treated. Only the webs and their associated foliage should be thoroughly covered.

Small nests can be pruned out of small to medium trees. Monitor trees early to detect the nests when only several leaves are involved. These small nests can be easily crushed. Do not burn or torch the nests in trees as this may do additional damage to the tree.Encourage Predators and Parasites – Over 80 species of parasites and predators have been identified in North America. Social wasps (yellow jackets and paper nest wasps), birds, predatory stink bugs and parasitic flies and wasps are the most important. Delay destruction of wasp nests until August when social wasps change from carnivores to sugar feeders. Try to withhold contact insecticide sprays until it is certain that predators and parasites are not present in sufficient numbers to control the webworms. Apply Bacillus thuringiensis (Bt) – The bacterial insecticide, Bt, is quite effective against fall webworms if it is applied when the larvae are small. Use formulations with UV protectants and thoroughly cover leaves next to nests. As these leaves are incorporated into the nest and eaten, the Bt will be ingested.

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