Asp Caterpillar (Southern Flannel Moth)

Megalopyge opercularis — the asp caterpillar (the larva of the southern flannel moth) — is one of the most painfully venomous insects in North America and a continuing public-health concern across the US South. The caterpillar is 25-35 mm long and entirely covered in long soft tan-to-blonde 'hair' that closely resembles a small piece of fur, cotton, or hair-tuft. The deceptive appearance is the source of the species' major risk: children, pets, and uninformed adults find the caterpillar irresistibly tuftable and routinely touch or pick it up. Hidden beneath the soft outer fur is a dense layer of hollow spines connected to internal venom glands. On contact with skin, the spines break off, embed, and inject a complex venom that causes immediate intense radiating pain often described as comparable to a bone fracture or major burn. The pain peaks within 5-10 minutes of contact, radiates from the contact site up the affected limb, and persists for 8-24 hours. Secondary symptoms include severe headache, nausea, vomiting, fever, lymph node swelling, and (in sensitive individuals) anaphylactic reactions requiring emergency medical attention. The Texas Department of State Health Services recommends seeking medical attention for any asp sting in a child under 12 or for any adult with significant systemic symptoms. Treatment is supportive (pain control, antihistamines, occasional epinephrine for anaphylaxis); commercial adhesive tape applied to the affected area and stripped off can remove embedded spines. The adult moth is small (35-40 mm wingspan), pale tan-to-cream-colored with cream-colored wings and dense fur, and looks like a piece of small flannel — the source of the 'flannel moth' name. The species is widespread across the US South from Florida to Texas, parts of Mexico, and into Central America.