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Fall webworms: an ‘aesthetic pest’

By SANDY LONG

UPPER DELAWARE REGION — The latest tree-loving caterpillar to raise alarm in the Upper Delaware region is Hyphantria cunea, or fall webworm, which constructs silky nests in trees in late summer. Due to the high visibility of the whitish-gray structures, the larval form of this insect stirs concerns about the impact of its eating habits. In reality, the hairy caterpillar is little more than an unsightly nuisance.

According to Douglas C. Allen, professor of forest entomology at the State University of New York, the fall webworm feeds mostly on alder, apple, beech, birch and oak, although in New York, it shows preference for black cherry. Brad Elison of the PA Bureau of Forestry reports the greatest impact in Pennsylvania to occur in black cherry, black walnut and apple trees.

Nests appear by early fall, and are constructed to encompass foliated sections of branches. Within the nest, which acts as a greenhouse to improve the larvae’s need for high humidity, the caterpillars feed on the foliage. The stringy structures are often enlarged to expand feeding opportunities. When several colonies exist on a tree, it can give the appearance that the tree is enshrouded by webbing.

No need for alarm

But despite the gruesome appearance of the nests, which become more bedraggled as the season wears on, the insects have little impact on the tree’s health since energy resources needed for survival and growth have already been produced by the trees. “Generally, fall webworms do little damage to the trees because of the time at which they’re defoliating,” Elison said.

By October, the larvae are finished with feeding, and drop to the ground, where they overwinter in soil, typically producing one generation per year in the region.

Although usually obvious every fall, during certain years the caterpillars are numerous enough to defoliate trees. Combined with this unattractive impact is the increased ability to sight the nests as early morning dew or fog lingers and lights the structures from within. Another reason for the nest’s high visibility is its builders’ penchant for sunny spots, typically common to roadways and forest openings, where people are more likely to see the nests.

Natural predators of the fall webworm are a variety of birds, rodents, insects and parasites. Small nests can be pruned out of small to medium trees and then crushed or burned. Nests should not be burned while they are still in the trees. The bacterial insecticide Bacillus thuringiensis (Bt) can be effective if applied when the larvae are still small.

TRR photo by Sandy Long
Fall webworms build nests to encapsulate foliage and rarely venture outside those nests until they have finished feeding and descend to the ground, where they overwinter. (Click for larger version)