The pine processionary (Thaumetopoea pityocampa) is a moth of the subfamily Thaumetopoeinae in the family Notodontidae, known for the irritating hairs of its caterpillars, their processions, and the economic damage they cause in coniferous forests.

The species is notable for the behaviour of its caterpillars, which overwinter in tent-like nests high in pine trees, and which proceed through the woods in nose-to-tail columns, protected from predators by their severely irritating hairs.

The Greek physician Dioscorides noted in the 1st century AD the urtication caused by the pine-dwelling pityokampē caterpillar, the name meaning "Aleppo pine larva" (Ancient Greek: πιτυοκάμπη; from πίτυς pitys "pine" and κάμπη kampē "caterpillar"),[2] in his 77 AD book De materia medica, Pliny the Elder recorded a treatment for the resulting itching in his Natural History; Theophrastus had earlier recommended the medicinal plant elecampane (horse-heal) in oil and wine to treat contact with "pine larvae" in his Historia Plantarum.

[5] The study has been cited innumerable times by inspirational and religious speakers who view it as a metaphor for blindly following a leader or for confusing activity with accomplishment.

Fabre considered his caterpillars to be mindless automatons, trapped because they were pre-programmed to blindly follow trails, in this case the endless one that they had laid down around the circular pot rim.

The caterpillars are readily recognised by their processionary habit and their presence (with large silken nests and signs of defoliation) in coniferous woods.

It has been spreading northwards since the 1990s, assisted by climate change and by commercial activities including planting of host trees and transportation, and has reached Brittany, forests to the north of Paris, and Strasbourg in northern France.

[7] A paler subspecies, T. pityocampa orana, occurs in North Africa from Morocco to Libya, at altitudes to 2,000 metres (6,600 ft) in the Middle Atlas; the adults fly between April and August.

[7] The larva is a major forest pest, living communally in large "tents", usually in pine trees but occasionally in cedar or larch, marching out at night in single file (hence the common name) to feed on the needles.

Larvae are parasitised by solitary wasps (Ichneumonidae, Braconidae) and some species of flies (Tachinidae), and may be infected by the processionary moth virus Smithiavirus pityocampae.

Simple contact with the hairs of the caterpillar can cause severe rashes (urticaria) and eye irritation in humans and other mammals,[11][13][14] and some individuals may have an allergic reaction.

[15] When stressed or threatened, fifth-stage larvae eject hairs shaped like harpoons, which penetrate all areas of exposed skin nearby and irritate them with an urticating protein.