Gypsy Moth Caterpillar (Spongy Moth)

The gypsy moth caterpillar, now commonly referred to as the spongy moth caterpillar (Lymantria dispar) following a 2022 renaming by the Entomological Society of America, is one of North America's most notorious forest defoliators. Introduced from Europe to Massachusetts in the late 1860s in a failed silk-production experiment, it has since spread across much of the northeastern and midwestern United States and parts of Canada.

This caterpillar is easily recognized by the rows of colored tubercles running down its back and the long tufts of bristly hairs covering its body. It feeds voraciously on the foliage of oaks and many other hardwood and some coniferous trees, and during periodic outbreak years, populations can explode to numbers dense enough to defoliate thousands of acres of forest in a single season.

Beyond its impact on forest health, the spongy moth caterpillar is ecologically significant as a food source for birds and predatory insects, and its irruptive population cycles are studied as a classic example of boom-and-bust insect dynamics tied to natural enemies like parasitic wasps and viral pathogens.

• Dark, hairy body covered in tufts of long bristles
• Distinctive double row of raised spots along the back: five pairs of blue spots toward the head, followed by six pairs of red spots toward the rear
• Grows to roughly 5-7 cm at full size
• Head is often mottled yellow and black
• Younger caterpillars are smaller and darker, becoming more distinctly marked as they mature
• Lookalikes: other tussock moth caterpillars, but the blue-then-red spot pattern is unique to this species

Spongy moth caterpillars are found in deciduous and mixed forests, parks, and residential shade trees throughout the northeastern and north-central United States and adjacent Canada, with oak being a preferred host alongside many other hardwoods such as birch, aspen, and willow. They are active from spring through early summer, feeding on foliage before pupating. Outbreak populations can appear cyclically, often every several years, driven by weather conditions and natural predator-prey dynamics.

Young caterpillars often disperse by "ballooning," spinning silk threads that carry them on the wind to new host trees. Older larvae feed at night and rest in bark crevices or leaf litter by day, a behavior that helps them avoid some predators. They feed on the leaves of a very wide range of tree species, and in outbreak years their combined feeding can defoliate entire stands of forest, though healthy trees often refoliate later in the season. They may release loose hairs or bite in defense. Birds, small mammals, and parasitic wasps and flies are important natural predators that help regulate populations.

Females lay a single mass of several hundred eggs covered in buff-colored hairs on tree trunks, branches, or other surfaces in mid to late summer; the eggs overwinter and hatch the following spring. Caterpillars pass through five to six instars over about seven weeks, feeding heavily on foliage as they grow. In early summer they pupate in a loose silk cocoon attached to bark or other shelter, emerging as adult moths after one to two weeks. Adult females are flightless and pale white, while males are smaller, brownish, and strong fliers that locate females by scent. The species produces one generation per year.