Invasion of the Spongy Moths...

A female spongy moth laying her egg masses.

Lymantria dispar — formerly known as the gypsy moth but now more commonly referred to as the spongy moth — has been especially evident this summer, however it can be observed year-round in Williamstown. This past winter, you might have noticed brown, squishy masses growing on the smooth bark of beech trees and other species. While they might  have appeared to be some sort of fungal infection, these were actually the egg sacks that give the spongy moth its name.

A spongy moth caterpillar, easily identifiable by its blue and red spots. (Photo by Bill McNee, Wisconsin Department of Natural Resources,

Come late spring, caterpillars (the larval state of the spongy moth) emerge from their egg sacks, and begin a three-month eating campaign against the returning spring foliage. In so doing, spongy moth caterpillars are responsible for many of the gaps observed in New England treetop canopies this summer, particularly in forests populated by high numbers of oaks and beeches, the leaves of which the caterpillars have a particular affinity for. The caterpillars — which grow up to two inches long and are recognizable by their parallel rows of blue and red tubercles (bumps) — can be spotted searching for food along the trunks and branches of these trees throughout the month of June.

A male spongy moth with its female counterpart. (Photo by Jerzy Strzelecki)

Here in Williamstown, we’ve noticed that the caterpillars have been particularly destructive up on Pine Cobble and near the New York-Vermont-Massachusetts tristate marker, although they’ve made their way down to the lower and upper loop trails of Hopkins Forest as well. Chances are you’ve been unable to avoid them this summer, as we’re seeing a boom in the spongy moth population, which typically happens every 10 to 15 years. By July, however, you’re more likely to see a dead caterpillar than an a live one: this is generally the time of year that individuals have begun to pupate or have otherwise been picked off by birds and diseases. While this current surge in the population may last for another year or two, we have hope that natural population controls will catch up soon enough. For now, though, we’re starting to finally see the moths themselves. The small, brown males can be seen flitting around oak trees, and the flightless females have already started to lay their eggs. By the end of July, the season of the spongy moth will have come to a close, with the eggs lying in wait to start the cycle again next May.

Two female spongy moths laying their egg masses, along with several cocoons and caterpillars. (Photo by Brodie Leo)


— Hopkins Forest Summer Caretaker Interns:  Ana Sofia Roldan ‘23.3, Brodie Leo ‘2025, Alice Lake ’25