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If you’ve been traveling around the Upper Delaware River region lately, you may have begun to wonder whether these early autumn days have taken a decided turn toward the time when ghouls and goblins roam the forests, draping filmy nets among silhouetted tree limbs to capture hapless victims. What you’re seeing, however, is not the work of witches, warlocks, or other Halloween-ish beings but the seasonal nests of the fall webworm.
Sometimes confused with the eastern tent caterpillar, the two can be differentiated by the location of their nests. The fall webworm constructs its nest at the ends of branches, while tent caterpillars build theirs in tree crotches. Both are pests of trees and shrubs.
As its name suggests, the fall webworm can typically be observed here from late summer through early fall. According to the Penn State College of Agricultural Sciences Department of Entomology, this native species feeds on more than 90 species of deciduous trees such as ash, birch, hickory, mulberry, walnut, poplar, elm, willow, cherry and crabapple.
After overwintering as a pupa in a cocoon sheltered by leaf cover and soil, or under bark flaps and in crevices, adults emerge throughout the summer and females deposit egg masses on the undersides of leaves. The adult moth covers the eggs with white hairs from her abdomen. Following hatching, the larvae spin small silken webs over the foliage that is their food source. They enlarge the nests as they grow, defoliating tree sections within the expanding nest.
This practice rarely kills a healthy tree, as it occurs toward the end of the growing season. Of course, it has a decided impact on the tree’s aesthetic value—unless you appreciate the fall webworm’s contribution to creating a creepier landscape as All Hallows’ Eve arrives again!
Otherwise, pruning away the nests is one way of managing the pest’s impacts while preserving the tree’s appearance. Natural predators of this species include birds, insects and parasitoids.