The Brown Tail Moth (Euproctis chrysorrhoea) requires visual identification due to the health risks associated with its larval stage. Understanding the insect’s appearance throughout its life cycle is crucial for managing its presence. The primary concern comes from the caterpillar’s microscopic, barbed hairs, which are toxic and can cause severe skin rashes and respiratory problems upon contact. Recognizing the moth, caterpillar, and its nests allows for timely action to avoid exposure.
Appearance of the Adult Moth
The mature Brown Tail Moth is a small to medium-sized insect, generally appearing in July and August, with a wingspan of approximately 40 millimeters (about 1.5 inches). Both male and female adults have wings and a midsection that are pure, bright white. This white coloration contrasts sharply with the moth’s most distinguishing feature: the dense tuft of reddish-brown or dark brown hairs located at the tip of the abdomen, which is the source of the moth’s common name.
Visual differences exist between the sexes. The female moth has a larger body and a much thicker, more pronounced brown tail tuft compared to the male. The male moth is slightly smaller and features larger, comb-like antennae, which it uses to detect female pheromones. Although the hairs on the adult moth are not toxic, their presence indicates the insect is ready to lay eggs.
Identifying the Caterpillar (Larval Stage)
The larval stage, or caterpillar, is the most recognized form of the Brown Tail Moth and the phase responsible for the public health hazard. Caterpillars are dark brown or blackish in color and can grow up to 30 to 40 millimeters long once fully developed. A distinctive, broken white stripe runs along each side of the body, providing a key identification marker.
Further distinguishing features include two conspicuous, reddish-orange spots (tubercles) positioned on the back towards the tail end of the larva. The entire body is covered in tufts of barbed, brownish hairs that readily break off and become airborne. These tiny hairs are microscopic, measuring about 0.15 millimeters, and contain a toxin that causes an intense, irritating rash. The hairs remain toxic in the environment for up to three years, making them hazardous even after the caterpillar is gone.
Egg Masses and Key Visual Markers
Female Brown Tail Moths lay their eggs in masses, typically on the underside of leaves belonging to host plants, such as oak, apple, or cherry trees. Each mass is covered with a thick, protective layer of brown or golden-brown hairs that the female sheds from her abdomen. These hair-covered clusters contain anywhere from 200 to 400 eggs and are laid in late July or August.
The characteristic winter web, or hibernaculum, is a key visual marker. These are communal nests where the young caterpillars overwinter, typically constructed at the tips of branches. The nest is a tight, silken structure, often incorporating a single leaf heavily wrapped in white silk, and is usually between two and five inches long. Unlike the looser, more centrally located webs of other common caterpillars, these tightly woven nests at the branch ends indicate a Brown Tail Moth infestation.