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Oak Processionary Moth

Oak Processionary Moth cluster

Oak processionary moth

The oak processionary moth (thaumetopoea processionea; OPM) is a pest which has established itself in oak trees in parts of the UK in recent years after being accidentally introduced from mainland Europe.

It damages oak trees, and can cause itching skin rashes and other health problems in people and animals.

Oak tree health

The caterpillars of OPM can damage our precious oak trees by feeding on the leaves. In large numbers they can strip whole trees bare of leaves, weakening the trees and making them vulnerable to other threats such as drought and disease.

Human and animal health

The caterpillars emerge about April every year and develop thousands of tiny hairs which contain an irritating substance called thaumetopoein. This substance can cause itching skin rashes, eye irritations and sore throats in people and animals who come into contact with them. In rare cases they can cause breathing difficulties and severe allergic reactions.

The hairs can be shed by the caterpillars as a defence mechanism, blown off by the wind, and left in the silken webbing nests which the caterpillars build on the trunks and branches of oak trees, sometimes at ground level. Nests can fall to the ground, and hairs can stick to the trunks and branches of oak trees.

What to do if you see OPM

If you see any OPM nests or caterpillars, do not touch or approach them. The caterpillars have a distinctive habit of moving about in or under oak trees in nose-to-tail processions, which gives them their name. The nests are white when new, but quickly become discoloured and harder to see.

The main risk period is April to July, when the caterpillars are active. However, avoid nests, even 'spent' nests, at any time, because the hairs in them can remain irritating for several months.

How to report OPM

If you see any OPM nests or caterpillars please report them to the main office of the green space you are visiting.

Burnham Beeches and Stoke Common: 01753 647358

The Commons: 01372 279083

City Gardens: 020 7374 4127

City of London Cemetary and Crematorium: 020 8530 2151

Epping Forest: 020 8532 1010

Hampstead Heath: 020 7332 3322

Highgate Wood: 020 8444 6129

Queen's Park: 020 8969 5661

West Ham Park: 020 8472 3584

Also report them immediately to the Forestry Commission, which is leading efforts to control its population, spread and impacts.

Go to the Forestry Commission website and use their Tree Alert on-line pest reporting form.

You can add photographs to your report, but do not risk contact to get a photograph.

If you cannot use Tree Alert, you may report them by email to the Forestry Commission or Tel 0300 067 4442.

Note that OPM does NOT live on walls, fences or other structures, and it does NOT build nests among oak leaves, so there is no need to report caterpillars or nests in these situations.

They also only attack other trees if they run short of oak leaves to feed on, so please do NOT report caterpillars or nests in other trees (not oak trees) unless there are stripped oak trees nearby.

What to do if you are affected by OPM

It is important to avoid contact with the hairs, to teach your children how to avoid them, and to protect your pets and livestock from them. Curious pets might need to be restrained from approaching nests and caterpillars.

However, if you are affected, the symptoms, although unpleasant, are not usually medically serious and will pass in a few days. You can ask a pharmacist for something to relieve the symptoms.

If you do have a serious allergic reaction, call NHS 111 or see a doctor. Similarly, consult a vet for badly affected animals.

Control programme

We are working with the Forestry Commission to survey for and treat affected oak trees and remove nests to minimise the population, spread and impacts of the pest. You can help us by reporting sightings as above. Details of the programme are available on the Forestry Commission website.

Further information

For detailed information about OPM, including videos, a guide to recognising it, a leaflet and poster, maps showing where it is known to be present, and guidance for tree surgeons and people who own oak trees in the affected areas, visit the Forestry Commission website    

26 April 2016
Last Modified:
04 September 2019