Fall Webworm Fact Sheet

The Fall Webworm, Hyphantria cunea (Drury), is a native of North America, and feeds on many species of deciduous forest, shade, and fruit trees. It is one of the few American insect pests that has been introduced into Europe and Asia. This pest is common all across Pennsylvania though it seems to have major outbreaks every few years.

Plants Attacked

In Pennsylvania, the fall webworm causes damage to approximately 90 species of deciduous trees. Preferred host plants include hickory (Carya), walnut (Juglans), birch (Betula), cherry (Prunus) and crabapple (Malus).

Insect Identification

There are two races of fall webworm in western Pennsylvania, the black-headed and the red-headed races. The larvae of the black-headed webworm are light greenish-yellow to pale yellow with two rows of black tubercles and the larvae of the red-headed webworm are tan in color with orange to reddish tubercles. Both are covered in long whitish hairs. The adult moth is about one inch long and ranges from pure white to white with a few black spots. The eggs are small, yellow or light green and are usually found on the underside of leaves.

Later instar fall webworm larva - photo by Eric Vorodi	Early instar larva of the black headed race of fall webworm

Life History

Winter	The pupae over-winter in cocoons in the ground. Pupae may also be found under loose bark and in leaf litter
Spring	Adults emerge from early-June through July and lay several hundred eggs that are deposited on the underside of a leaf (the blackheaded webworm deposit its eggs in a single layer and the redheaded in a double layer). Scales from the female’s abdomen lightly cover the eggs, which hatch a week later. The larvae immediately spin webs and feed on the enclosed leaves.
Summer	The larvae mature in about six weeks, at which time they drop to the ground to pupate.
In southern Pennsylvania there are two generations each year.

Damage Symptoms

The larvae spin unsightly light grey webs, starting at the tips of the branches and slowly extending down the branch towards the trunk, feeding on the foliage enclosed by their webs. Damage to the host plant is primarily aesthetic, since leaves are usually eaten late in the season when it is not usually a threat to the health of the tree.

Fall webworm netting in a chokecherry	Fall webworm damage on crabapple

Management Options

Biological	The webworm has many predators and parasites. Among them are social wasps, birds, predatory stink bugs and parasitic flies and wasps. The bacterial insecticide ‘Bacillus thuringiensis’ is effective if applied when the larvae are small.
Mechanical	On small trees, nests may be cut out and destroyed.
Chemical	If detected early, insecticide applications should be applied to the nest and nearby foliage. Tall trees may be treated systemically. Control Measures for Fall Webworm

 

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