The asp caterpillar, often called the puss caterpillar, is a creature known less for its transformation and more for its striking, deceptively soft appearance. This inch-long larva is covered in a thick coat of hair, resembling a tuft of cotton. The seemingly cuddly look, however, conceals one of the most potent stinging mechanisms found in North American caterpillars. Its notoriety stems entirely from the intense, localized pain it can inflict upon contact.
The Identity of the Asp Caterpillar
The asp caterpillar is the larval stage of the moth species Megalopyge opercularis, part of the family Megalopygidae, commonly known as the flannel moths. The common names for the larva are numerous, including puss caterpillar, woolly slug, and tree asp. This caterpillar typically grows to about one to one-and-a-half inches long and has a plump, teardrop-like body shape. Its dense covering of long, silky hair conceals its head and legs when viewed from above. The color of this furry coat is variable, ranging from a grayish-white to golden brown or sometimes a dark, charcoal gray.
The Adult Form: Puss Moth
The asp caterpillar completes its metamorphosis into the Southern Flannel Moth, which is also sometimes simply called the Puss Moth. The adult moth is equally fluffy, covered in a thick, flannel-like coat of scales and fine hairs that give it a stout-bodied appearance. The wings of the moth are generally muted in color, often a mix of creamy white, brown, or black. The adult moth is nocturnal and rarely encountered by humans. The Southern Flannel Moth is completely harmless and lacks the stinging apparatus of the caterpillar. Its primary biological function is reproduction, and it has a relatively short lifespan of only about five to seven days.
Understanding the Life Cycle Stages
The life cycle of Megalopyge opercularis involves complete metamorphosis, beginning when the female moth lays batches of eggs on the leaves or twigs of host trees. The larva emerges and feeds on various deciduous trees like oak, elm, and maple. This caterpillar stage can last for several weeks, during which it may molt up to seven times. The species generally produces two generations each year, with the highest numbers of the stinging caterpillars typically observed in the summer and a second peak in the fall. Once fully grown, the larva spins a dense, tough, brownish-gray silk cocoon, often attached to a twig or bark, where it pupates and spends the winter before the adult moth emerges in the late spring.
The Medical Concern of the Sting
The puss caterpillar’s sting is a severe envenomation caused by a North American insect. The venom is delivered by hollow, glass-like spines called setae, which are hidden underneath the soft outer coat of hair. When the caterpillar is pressed against the skin, these spines break off and release venom directly into the wound, causing intense, throbbing, radiating pain that spreads up the affected limb. This sensation is often described as feeling similar to a broken bone or a severe burn, and systemic symptoms like headaches, nausea, and fever may occur. For immediate care, the hidden spines should be removed by applying and quickly peeling off a piece of adhesive tape, followed by applying an ice pack or cold compress.