The caterpillars (larvae) of oak processionary moth (OPM) are pests of oak trees (trees in the Quercus genus), and are a hazard to human and animal health. This page contains information about OPM including how to recognise the caterpillars and how to report a sighting.
Why is OPM an issue?
OPM caterpillars feed on the leaves of several species of oak trees. Large populations can strip whole oak trees bare, leaving them more vulnerable to other pests and diseases, and to other stresses, such as drought. Older caterpillars develop tiny hairs containing an irritating protein called thaumetopoein.
Please do not touch OPM larvae or caterpillars, or disturb any nests.
The caterpillars can shed hairs when threatened or disturbed. The hairs can be blown by the wind and they accumulate in the caterpillars’ nests, which can fall to the ground. They can stick to trunks, branches, grass and clothing as well as to equipment, such as ropes, used by tree surgeons and forestry and ground-care workers.
Contact with OPM caterpillar hairs can cause itching skin rashes and eye irritations, as well as sore throats and breathing difficulties in people and animals. The risk of exposure to these hairs is highest in May and June.
Among the groups most vulnerable to the health hazards are:
- curious children;
- curious pets;
- people who work on or close to oak trees;
- anyone spending time close to infested trees; and
- grazing and browsing livestock and wild animals.
If you do come into contact with the larvae or caterpillars by accident, please follow the health advice on the NHS website. If you have an itching skin rash and/or conjunctivitis or other symptoms, contact your GP, or call NHS Direct on 111. The call is free from any phone.
Where is OPM found?
OPM is only known to be established in a relatively small geographical area of the country across London and surrounding areas. However it is found in Epsom & Ewell.
How to spot OPM
OPM caterpillars have the following distinguishing characteristics:
- 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.
- They live and feed almost exclusively on oak trees. They can sometimes be seen processing across the ground between oak trees.
- They 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.
- They are only seen in mid- to late spring and early summer (May, June and July).
- They have very long, white hairs which contrast markedly with the much shorter, almost undetectable, irritating hairs.
- They have a grey body and dark head. Older larvae have a central dark stripe with paler lines down each side.
- They are not usually found on fences, walls and similar structures, such as garden furniture – these tend to be other, similar-looking caterpillars.
Please report any sightings of OPM to the Forestry Commission. You can do this online, by email or by telephone:
Reporting your sighting online:
- use the Forestry Research's Tree Alert online reporting form. (Forest Research is the research agency of the Forestry Commission).
If you are unable to use the form, please use one of the following contact options:
- email your report to email@example.com
- telephone it to 0300 067 4442, providing as much detail as to the location as possible.
The Forestry Commission does not fund control, this is up to the tree owner and not a legal requirement.
Take a look at the Forest Research website for more information. You can also view the distribution of the Oak processionary moth (Thaumetopoea processionea) in Surrey on the Forest Research website.