Oak processionary moth: what the agencies record

This page reports what the official source says. It is not legal advice: ask the office named here. It describes what the named sources say and is not a diagnosis of any tree or medical advice. Each section names its source, all read on October 9, 2026. The zone table and the legal status are on the oak processionary moth regulated status page and are not repeated here. The moth has a row in insect pests of trees. Abbreviation: OPM is oak processionary moth.

Names and history

Forest Research gives the scientific name Thaumetopoea processionea and lists the moth as present in the United Kingdom and notifiable. It says OPM was first accidentally introduced to England in 2005. The Forestry Commission's GOV.UK guidance, last updated August 13, 2026, says it was first identified in London in 2006 and spread to some surrounding counties in south east England. The two pages give different years, so each is credited to its own source. Forest Research says it is established in southern, central and western Europe as far north as Great Britain, northern Germany and the Netherlands.

Hosts

Forest Research says the caterpillars feed on the leaves of oak, the genus Quercus. Its manual page on biology says both of Britain's native oaks, pedunculate and sessile, are susceptible, and gives a broadly descending order of susceptibility that starts with Turkey oak and English or pedunculate oak and ends with sessile oak and cork oak. It says the caterpillars take other broadleaved trees, and names sweet chestnut, hazel, beech, birch and hornbeam, usually only when oak leaves run short, and that they cannot usually complete their life cycle on trees that are not oak. Large populations can strip whole oaks bare, which leaves trees more vulnerable to other pests, diseases and stresses such as drought. The damage leaves leaves skeletonised with the main veins remaining.

Caterpillars, nests and moths as described

Forest Research says the caterpillars move in nose-to-tail processions in late spring and early summer, often arrow-headed, and cluster while feeding. They have a grey body, a dark head and very long white hairs, and older ones show a central dark stripe. GOV.UK says they are about 2 mm long on emerging in spring, develop the irritating hairs at 1 cm, and are fully grown at 2 cm.

Forest Research describes the nests as white silken webbing on the trunks and branches of oak trees, anywhere from ground level to high in the tree, and almost never among the leaves. GOV.UK describes fresh nests as white, with silken trails along the branches and leaves. GOV.UK gives dome or teardrop shapes from the size of a golf ball to a rugby ball, and says the nests turn brown and can stay attached for many months after the moths have emerged. Forest Research adds hammock and blanket shapes, a size from a few centimetres to several feet across, and says nests on fences, walls and other shrubs are usually made by harmless species. It describes the adult as an undistinctive brown moth, not a health hazard, for which the plant health authorities do not require reports.

Life cycle as the sources state it

Forest Research's biology page gives one generation a year. Moths emerge, mate and lay eggs between late July and sometimes the end of September, in plaques 2 to 3 cm long on high branches and twigs that stay through autumn and winter. Caterpillars emerge from the eggs from about mid-April to mid-May after the winter, and from as early as March in warm weather. They pass through six stages, L1 to L6, and the hairs begin at about L3. Older caterpillars feed mainly at night and rest in the nests by day. From late June to early August they pupate in the nests, and the moths emerge from about the middle of July to late September. Both sexes live three to four days as adults. Males can fly up to 20 km, while most females stay near the nest.

Health hazard as the sources state it

Forest Research says contact with the hairs can cause itching skin rashes and eye irritations, and sore throats and breathing difficulties in people and animals, from an irritating protein called thaumetopoein. Its manual page says a fully grown caterpillar can carry thousands of barbed hairs, that the greatest risk is May to July, and that hairs can be present on old nests and blown or touched at any time of year. The hairs can stick to ropes and equipment used by tree surgeons, forestry workers and ground-care workers. GOV.UK adds that repeated exposure can sensitise a person and worsen symptoms. Pets and grazing livestock are named as especially exposed.

Zones and reporting

Forest Research says OPM is established in most of Greater London and some surrounding counties, that the largest outbreak cannot be eradicated, and that a government programme has run since 2012 to slow spread and reduce impact. The rest of the United Kingdom is designated an area free from the pest. GOV.UK describes three management areas, the established area, the buffer zone (which contains a 10 km wide exclusion zone with extra movement restrictions) and the area free from the pest, and says that for suspected nests or caterpillars the report goes through TreeAlert, which GOV.UK names as the preferred route. In the buffer zone and the free area the Forestry Commission says a Statutory Plant Health Notice may be issued and that it takes charge of treating infested trees; in the established area landowners manage OPM on their land. See report a pest in the United Kingdom for the reporting offices.

The Forest Research manual

Forest Research hosts the Oak Tree Owners' OPM Manual in 12 parts. This page draws on parts 2 and 4 only. What each part covers is set out on the OPM manual guide. The manual's control and removal parts are not summarised here, and GOV.UK recommends that a professional carry out nest and caterpillar removal, calling it hazardous.

Text on the United Kingdom is adapted from Forest Research and GOV.UK and contains public sector information licensed under the Open Government Licence v3.0.