Pine Processionary Caterpillar

Thaumetopoea pityocampa — the pine processionary caterpillar — is one of the most behaviorally remarkable and most medically significant Lepidoptera in the world. The species is native to the Mediterranean basin (southern Europe, North Africa, the Levant) and has been expanding northward in Europe since the 1990s as climate warming reduces the lower temperature limits of overwintering survival. Adult moths emerge in midsummer, mate, and lay eggs on pine needles; caterpillars hatch and live communally inside silken tents constructed in the host pine canopy. As the larvae mature in autumn and winter, they feed on pine needles at night and aggregate in tents during the day. In late winter / early spring, fully mature caterpillars descend the pine trunk in single-file head-to-tail processions to find soil suitable for pupation. The processions can include dozens to hundreds of individuals, with the leader choosing the route and every following caterpillar walking in lockstep behind, touching the rear of the caterpillar in front. Procession behavior is mediated by silk trails, tactile contact, and chemical cues; the caterpillars reorganize the chain at obstacles and re-form if disrupted. The procession behavior is the source of the species' common name and one of the most-cited examples of collective behavior in caterpillars. The medical importance is severe: the caterpillar's body is covered in urticating bristles containing the protein thaumetopoein, which causes immediate intense itching dermatitis on contact, allergic and anaphylactic reactions in sensitized individuals, and severe ocular and respiratory reactions in close encounters. Dogs are particularly at risk: bites and licking can cause tongue necrosis (sometimes requiring amputation) and have killed dogs across the Mediterranean. The species causes significant pine forest defoliation and is a continuing public-health and veterinary concern across southern Europe.