Oak Processionary

Oak Processionary (Thaumetopoea processionea) is a moth originally found only in southern Europe. Starting in the late twentieth century, this caterpillar also occurs also occurs in the UK.
For both the tree (caterpillar eats leaves) and humans, the oak processionary is a pest. The caterpillar is covered with long white hairs and very small (0.2 to 0.3 mm) defensive bristles. The defensive bristles are arrow-shaped and have barbs; the caterpillar “shoots” the defensive bristles away at impending danger. The defensive bristles can enter the eyes, respiratory tract and skin where they cause allergy-like symptoms. The defensive bristles are spread by the wind so that passing hikers, cyclists and dogs can come into contact with them.
Oak is the preferred food source of the oak processionary, but the leaves of hazel, hornbeam, sweet chestnut, birch and beech are also eaten. Oak is the host plant of the oak processionary: the moth lays its eggs at the top of trees in September. In spring, the eggs hatch at the same time as the leaves, and the larvae begin to feed on the leaves. The caterpillars’ nests consist of webbings and droppings, defensive bristles and old skins (from molting). At night, the caterpillars go in a long thick line – “in procession” – in search of other oaks; during the day, they remain in the nest.
Great tits, parasitic wasps and parasitic flies are among the natural enemies of the oak processionary.