The caterpillars are mostly found in oak trees or on the ground under them in late spring and early summer, and do not live on fences, walls, etc. as other caterpillars do. They have very long, white hairs contrasting markedly with shorter hairs. The caterpillars of several other species may be mistaken for the oak processionary. The moths pose an increasing nuisance to humans as their range is extended. The backs of older caterpillars (3rd to 6th instars) are covered with up to 63,000 pointed defensive bristles, sized between 0.2 and 0.3 millimeters, which contain an urticating toxin, the protein thaumetopoein. The setae break off readily, become airborne and can cause epidemic caterpillar dermatitis (lepidopterism), manifested as a papular rash, pruritus, conjunctivitis and, if inhaled, pharyngitis and respiratory distress, including asthma or even anaphylaxis. It has been found that the skin irritation and itching caused by contact with these hairs can be largely eliminated by the use of cetirizine-based antihistamine tablets. Transmission of the hairs can be airborne, by ground contact via plants or grass or even by water contact in stillwater e.g. garden ponds. The toxicity of the hairs remains active beyond the normal life cycle of the moth and in some cases can remain a problem for several seasons. Mowing a lawn can bring a person into contact with these hairs. One alternative is to adopt a grass mulching technique to reduce possible contact, and to speed up the biological breakdown of the irritant hairs. Large populations can strip trees bare, leaving them weakened and vulnerable to other threats. Nests can be removed, or the caterpillars sprayed with pesticides soon after they hatch. However, neither approach is 100% effective. Male moths can be trapped in pheromone traps; this does not significantly reduce the population, but provides an indication of moth distribution. The caterpillars were accidentally introduced to the UK in 2005, almost certainly as eggs on live oak plants imported from continental Europe. Later distribution of the pest probably arose from several similar introductions, in addition to spread from the original point of introduction. By 2019 they had spread to all 33 London boroughs, and the Government had spent £37 million trying to control them. The general public have been asked to look out for these caterpillars and to report them, rather than deal with them themselves.