Gypsy Moth Caterpillar

The gypsy moth caterpillar, now increasingly referred to by the common name spongy moth caterpillar, is the larval stage of an invasive moth introduced to North America from Europe in the late 1800s. It has since become one of the most significant defoliators of hardwood forests in the eastern United States and parts of Canada, periodically erupting into large-scale outbreaks that can strip broad swaths of forest canopy.

Caterpillars hatch from egg masses in spring and feed voraciously through several molts, favoring oak but also accepting a very broad range of other hardwood and, to a lesser extent, softwood tree species. During outbreak years, populations can build to extremely high densities, with dense clusters of caterpillars visible on tree trunks and branches during the day.

Outbreaks tend to be cyclical, often collapsing after a few years due to the combined effects of a naturally occurring virus, a fungal pathogen, and buildup of natural predators and parasites, followed by a period of lower population levels before the cycle repeats.

• Young larvae are dark and hairy; mature larvae reach up to about 2.5 in (65 mm), with a mottled grayish body covered in tufts of long hairs
• Distinctive double row of raised spots down the back: five pairs of blue spots toward the head followed by six pairs of red spots toward the tail
• Head capsule is pale with dark markings
• Adult male moth is brown with dark wavy bands on the forewings and a feathery antennae; adult female is larger, whitish, and nearly flightless
• Egg masses are buff to tan colored, covered in a felt-like layer of hairs, and are laid on tree trunks, branches, or other sheltered surfaces

Established across the northeastern and midwestern United States and parts of southeastern Canada following its introduction from Europe, with populations gradually expanding into new regions over time. It is found in deciduous and mixed forests, especially those with abundant oak, as well as in suburban and park settings with mature hardwood trees, with larvae active from spring through early summer.

Young larvae often disperse from their hatching site by ballooning on silk threads carried by the wind, while older larvae feed at night and shelter in bark crevices or leaf litter by day, a behavior that helps them avoid some daytime predators. As a broad generalist feeder, the species can defoliate hundreds of species of trees and shrubs during outbreak years, with oak, aspen, and birch among its preferred hosts. Population booms are eventually checked by a specific nucleopolyhedrosis virus and an introduced fungal pathogen, along with predation by birds, small mammals, and parasitic insects, producing the characteristic multi-year outbreak-and-collapse cycle.

Development is complete metamorphosis with one generation per year. Eggs are laid in a single mass in mid- to late summer and overwinter, hatching the following spring as host trees leaf out. Larvae pass through several instars over roughly six to eight weeks, feeding increasingly at night as they mature, before pupating in a sheltered spot such as bark crevices or leaf litter. Adults emerge in mid- to late summer, and after mating the flightless females lay a new overwintering egg mass, completing the annual cycle.