The oak processionary caterpillar is the larva of a moth, which lays its eggs mainly in the tops of oak trees where they hibernate. The eggs hatch from mid-April to early May. Groups of caterpillars come together after a number of molts. Oak processionary caterpillars form large nests on the trunk and side branches of the oak tree. The caterpillars molt three times, then they get stinging hairs.
The stinging hairs cause health problems. Oak trees with oak processionary caterpillars can be recognized by the spherical, white-gray nests on the trunks or thicker branches. The nests are dense spindles of molting skins, with stinging hairs and feces.