Understanding Fall Webworms

Fall webworms in a Tennessee forest. Photo credit: UF IFAS Extension

Have you ever noticed the big knots of webbing in some trees this time of year? They’re usually sort of a brown-pink hue, much too big for a spider but not really tightly wound enough to be a nest or cocoon.

A close-up of the fall webworm tent reveals dozens of tiny caterpillars and skeletonized leaves. Photo credit: Carrie Stevenson, UF IFAS Extension

What you’re seeing actually is a cocoon of sorts—a big shared one created by hundreds of larval fall webworms (Hyphantria cunea). This common name is a bit of a misnomer. The webbing—often referred to as a tent—is built in late summer, not fall, and these are caterpillars, not worms. But I suppose it gets the idea across!

On a recent hike, I saw webbing up close, so I was able to really investigate the caterpillars inside. They are quite small, and will undergo up to five molting stages, or instars, before adulthood. Upon transformation, they will become white or speckled moths. Fall webworms are native to the entirety of the United States—in the northern end of their range the moths will be solid white, whereas further south they will have darker spots on their wings. Due to accidental introduction, fall webworms are invasive throughout Europe and portions of Asia.

Adult fall webworm, with spots on white, which is typical for members of this species from the southern part of its range. Photo credit: Lyle Buss, UF

Host plants include a wide range of more than 80 deciduous hardwood species, allowing a broader spectrum of places for the caterpillars to undergo their various larval phases. During their months in the “tent”, the larvae feed on and skeletonize the leaves encapsulated within their webbing. This causes damage to the leaves, but results in no permanent issues to the trees—being late summer and fall during their tenure, the trees eventually lose their leaves anyway.

While the tents are a bit unsightly, controlling the caterpillars or removing the webs is unnecessary. Once they’ve reached their final caterpillar stage, they’ll hide out in tree bark and leaf litter on the ground until metamorphosizing into moths the following spring. Adult moths mate and lay eggs in the late spring and summer, starting the cycle all over again.