Last Friday morning this odd procession of caterpillars crossed the street in front of the house. From far it looked like a snake. But a closer look revealed something more unusual: An insectoid train of several hundred caterpillars.
This whole line of caterpillars was about 2.1m (7 ft) long and parted after a couple of minutes into to independent lines to circumvent an obstacle and later the two lines merged back together later. For the caterpillars it seemed to be a real social and disciplined undertaking to cross the road. The caterpillars appeared to be Thaumetopoea pityocampa (Thaumetopoeidae), also known as the Pine Processionary Caterpillar. It is poisonous with stingy hairs that cause skin irritation. It lives off living pine trees, eating vast amounts pine needles and defoiling whole trees. It is also one of the most social caterpillars according to T.D Fitzgerald at the State University of New York College at Cortland.
According to studies these caterpillars are active at night and stay in their social nests during growth, get out to seek food and return to the nests at day. When the caterpillar is fully grown they abandon their nest and seek soft soil to go into pupal stage until they turn into the large, nasty, hairy pine processionary moth.
These insects can cause serious damage to young pine trees and it's considered one the most destructive conifer pests. It's a good idea to clear them but avoid any contact with their hairs.