They have a distinctive habit of moving about in late spring and early summer in nose-to-tail processions, from which they derive their name. The processions are often arrow-headed, with one leader and subsequent rows of several caterpillars abreast;

Live and feed almost exclusively on oak trees. They can sometimes be seen processing across the ground between oak trees;

Will usually only affect other broad-leaved tree species if they run short of oak leaves to eat – they have been observed feeding on sweet chestnut, hazel, beech, birch and hornbeam. However, they generally cannot complete their development on other tree species;

They cluster together while they are feeding on oak leaves and moving from place to place;

Are only seen in mid- to late spring and early summer (May, June and July);

Have very long, white hairs which contrast markedly with the much shorter, almost undetectable irritating hairs;

Have a grey body and dark head. Older larvae have a central dark stripe with paler lines down each side;

Are not usually found on fences, walls and similar structures, such as garden furniture.

Tree Health OPM caterpillars can be a hazard to oak trees because they feed on the leaves. Large populations can strip trees bare, leaving them vulnerable to other pests and diseases and less able to withstand events such as drought and flood. The Health Risk to Humans The main concern with this pest is the threat to human health because the caterpillar hairs can cause severe allergic reactions. The caterpillars have thousands of tiny hairs which can be blown about by the wind. These hairs contain an irritating substance called thaumetopoein. Contact with the hairs can cause itchy skin rashes, eye and throat

By Haadi