The spongy moth, Lymantria dispar, is an invasive insect that poses a significant threat to North American forests. Understanding its appearance at each life stage is important for early detection and management efforts. Accurate identification helps slow its spread and mitigate damage to trees and ecosystems.
Adult Moth Characteristics
Adult spongy moths exhibit distinct differences between males and females in size, coloration, and flight capability. Male spongy moths are generally smaller, with a wingspan of approximately 25 to 30 millimeters. They have a brownish-gray body with darker, wavy markings across their wings. Their antennae are notably feathery, used to detect female pheromones. These males are strong flyers, actively searching for females.
In contrast, female spongy moths are typically larger, with a wingspan ranging from 50 to 60 millimeters. They have a cream-colored or off-white body and wings, often displaying faint dark zigzag patterns. Unlike males, female spongy moths are heavy-bodied and flightless, despite having fully developed wings. They remain near their pupal cases after emergence, emitting pheromones to attract males.
Caterpillar Features
The spongy moth caterpillar is the most destructive life stage, responsible for extensive tree defoliation. These larvae can grow up to 6 centimeters (about 2.5 inches) in length when fully mature. Their bodies are covered with numerous hairs. A key identifying characteristic is the distinctive pattern of colored spots along its back. It features five pairs of blue spots followed by six pairs of rusty-red spots running down its segmented body. The head capsule is typically yellow with black markings, and the body hairs can sometimes cause skin irritation upon direct contact.
Egg Masses and Pupal Stage
Spongy moth egg masses are often the most visible sign of an infestation, particularly during the dormant season. These masses are tan to yellowish-brown and have a fuzzy, velvety texture, resembling a small piece of sponge. They typically measure about 2 to 4 centimeters long and can contain hundreds of eggs. Females deposit these egg masses on various surfaces, including tree trunks, branches, rocks, outdoor furniture, and vehicles, often in protected crevices.
The pupal stage follows the caterpillar phase, with pupae appearing dark reddish-brown and having a hard, shell-like exterior. They are often found in sheltered locations, sometimes loosely attached by silken threads, and adult moths typically emerge from these pupae in mid-summer.
Similar-Looking Insects
Distinguishing spongy moths from similar-looking insects is important for accurate identification. The fall webworm caterpillar, for instance, is often confused with the spongy moth caterpillar. Fall webworm caterpillars construct large, unsightly silk webs at the ends of tree branches, enclosing their feeding area. Spongy moth caterpillars, however, do not create such extensive communal webs; they feed openly on leaves.
Another species often mistaken for the spongy moth caterpillar is the forest tent caterpillar. While both are hairy and can defoliate trees, the forest tent caterpillar has a row of white, keyhole-shaped spots down its back, rather than the distinct blue and red spots characteristic of the spongy moth caterpillar. Forest tent caterpillars often congregate in silken mats on tree trunks or branches, but they do not form large feeding webs like the fall webworm.