The organism formerly known as the Gypsy Moth is now officially referred to as the Spongy Moth (Lymantria dispar). This invasive species is known for its destructive larval stage, which feeds voraciously on the leaves of a wide variety of trees and shrubs. Understanding the visual characteristics of the caterpillar is the most effective way to identify this pest. The caterpillar stage is the longest and most recognizable phase of the life cycle.
Distinctive Physical Characteristics
The mature Spongy Moth caterpillar is large, reaching 1.5 to 2.5 inches (3.8 to 6.4 cm) when fully grown. Its body is generally dark, mottled gray or yellow, and covered with numerous tufts of stiff, bristle-like hairs, which can sometimes cause skin irritation. The caterpillar also possesses a distinct, pale yellow head capsule with dark markings.
The most reliable feature for identification is the pattern of raised, colored bumps, called tubercles, that run in two rows down the length of its back. Near the head, the caterpillar displays five pairs of blue tubercles. These are followed by six pairs of brick-red or orange tubercles extending toward the tail.
This unique arrangement of five pairs of blue spots followed by six pairs of red spots distinguishes the Spongy Moth caterpillar from other common species. The caterpillar’s overall appearance is robust and densely covered in hair. Unlike some similar species, it does not spin a silken tent or web.
Changes in Appearance During Growth
The caterpillar’s appearance changes significantly as it develops through its larval stages, known as instars, which typically involve five or six molts. When the larvae first hatch in the spring, they are very small, measuring only about one-sixteenth of an inch and are dark brown to black. At this early stage, they are covered in long, fine black hairs and may not yet display the characteristic colored dots.
Newly hatched caterpillars often climb to the top of their host tree and spin silken threads, a behavior known as “ballooning.” The wind catches these silk strands, allowing the larvae to disperse to new locations. As the caterpillar grows and molts, the distinctive colored tubercles begin to appear, becoming clearly visible around the third or fourth instar.
Once they reach this later stage, the caterpillars are much larger and begin to exhibit different behaviors, such as feeding primarily at night. During the day, these larger larvae tend to crawl down the tree trunk to hide in sheltered, dark places like leaf litter or under bark.
Identifying the Egg Mass and Pupa
Identifying the Spongy Moth at the egg mass or pupal stage tracks its presence outside the spring and summer feeding season. The egg mass is the overwintering stage, typically laid in late summer and remaining until the following spring. These masses are one of the most recognizable signs of the moth.
The egg mass has a fuzzy, felt-like, or “spongy” texture, which is the source of the moth’s common name. It is covered with a layer of hair, ranging from tan to buff or yellowish, deposited by the female moth from her abdomen. Egg masses are generally oval-shaped, about 1 to 3 inches long, and may contain anywhere from 100 to 1,000 eggs.
The pupa is the non-feeding, immobile stage that follows the caterpillar stage, typically occurring in mid-summer. It appears as a dark, reddish-brown, hard, shell-like casing. Lacking a silken cocoon, the teardrop-shaped pupa is often found in sheltered spots, sometimes lightly attached by silk threads to tree trunks, under branches, on outdoor furniture, or in crevices.