The spongy moth caterpillar, now the official common name for the insect previously known as the gypsy moth in North America, is a widely recognized defoliating insect. Accurate identification is important for monitoring and managing their populations. These caterpillars feed voraciously, and their presence can lead to considerable leaf loss, which may weaken trees and make them vulnerable to other environmental stressors.
Distinctive Physical Characteristics
Spongy moth caterpillars have several defining features. When fully grown, they typically measure between 1.5 to 2.5 inches long, though some female caterpillars can reach up to 3 inches. Their bodies are generally dark, appearing mottled yellow to gray, or even brownish-yellow and black.
A prominent feature is the dense covering of bristly hairs across their bodies, which can sometimes cause skin irritation upon contact. Spongy moth caterpillars possess a unique pattern of five pairs of raised blue spots, followed by six pairs of raised red spots, running along their backs. Their head capsule is typically yellow with distinct black markings.
Identifying Similar Caterpillars
Several other caterpillar species might be confused with the spongy moth caterpillar, but they have distinct differences. The Eastern Tent Caterpillar, for instance, has a prominent white or yellowish stripe down its back, flanked by small blue spots on its sides. Unlike spongy moth caterpillars, Eastern Tent Caterpillars construct noticeable silken tents in tree branches.
The Forest Tent Caterpillar, despite its name, does not build a silk tent. It is distinguished by a row of white, footprint-shaped or keyhole-shaped marks down its back, accompanied by light blue stripes along its sides. To differentiate these species from the spongy moth caterpillar, look for the unique combination of five pairs of blue dots followed by six pairs of red dots, a pattern found only on the spongy moth caterpillar. Spongy moth caterpillars do not produce communal webs or tents.
Where and When to Find Them
Spongy moth caterpillars are typically encountered from late spring to early summer. Eggs hatch around April or May, often coinciding with tree budding. The larval feeding stage lasts approximately five to eight weeks, with peak activity during May and June. Younger caterpillars may disperse by “ballooning,” floating on silk threads carried by the wind.
These caterpillars feed on the foliage of a wide range of trees, preferring oaks, but also consuming leaves from maples, birches, aspens, willows, apple, and crabapple trees. In areas with high populations or scarce preferred food sources, they may also feed on conifers such as pine, spruce, and hemlock. Older caterpillars typically feed at night, seeking shelter during the day in bark crevices or leaf litter at the base of trees. Their feeding often results in visible defoliation, where leaves are chewed, and can be accompanied by small fecal pellets, known as frass, beneath infested trees.