The adults have grey forewings with white and darker grey markings and a wingspan of ca. 30 mm. The adults emerge and fly between the end of July and mid September. The eggs (with up to 300 eggs per batch) are laid in contiguous rows along one to two year old twigs and covered by hairs from the female.
The first instar larvae overwinter within the eggs and hatch in mid- or late April. They are nocturnal and feed gregariously. The newly emerged larvae are brown. In later instars, their body becomes grey. From the third instar onwards, each larva starts producing thousands of very small, barbed urticating hairs (ca. 0.1 mm). These hairs contain an allergenic protein, thaumetopoein; which can be actively released in the air when the larvae are disturbed. After moulting, the previous stage's hairs remain on the exuvia, but can become airborne and are still allergenic. In dry weather, these urticating hairs can remain allergenic for months. From the 5th instar on, the larvae start building silken nests at the base of lower branches, on the trunks or at the base of the trunks, which may contain thousands of individuals. The larvae usually stay in their nests during the day leaving at night in a characteristic procession to feed in the canopy of the trees. If the leaves on a tree become scarce, the larvae move away from the tree collectively to colonise another host nearby. At pupation time (late June, early July), the larvae spin cocoons inside the nests.