Oak processionary moth

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

Leisure background

OPM procession_H KuppenOPM cluster_Crown copyrightForestry Comission







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

OPM caterpillars damage oak trees by feeding on the leaves. Large numbers can strip whole trees almost 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 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 or close to 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 silken webbing nests are white when new, and often have silken trails leading to them. They quickly become discoloured and harder to see against the dark colour of oak tree bark.

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 many months. Nests can sometimes fall to the ground.

How to report OPM

Please report sightings on Guildford Borough Council owned land to Parks & Leisure Services by emailing parks@guildford.gov.uk or phoning 01483 444 718.

Please report sightings on other land in the borough to the Forestry Commission using their Tree Alert online pest reporting form, which you can access at the Forestry Commission website. You will have to add a photograph to your report, but do not risk contact to get a photograph.

It would be courteous to contact the landowner if you know who they are.

If you cannot use Tree Alert or get a photograph, you may report them by email to opm@forestrycommission.gov.uk or by phoning 0300 067 4442.

Note that OPM:

  • does NOT live on walls, fences or other structures

  • it does NOT build nests among oak leaves

so there is no need to report caterpillars or nests in these places.

They occasionally attack other trees, but only if they run short of leaves to feed on, so please do not report caterpillars or nests in other 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 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 NHS111 or see a doctor. Similarly, consult a vet for badly affected animals.

If you own oak trees

If you have OPM in your own oak trees, you will be issued with a Statutory Plant Health Notice by the Forestry Commission, requiring you to have the pest removed. This does not mean you are in any trouble - the notices are a management tool.

Guildford Borough lies in the OPM Control or Buffer Zone where the Forestry Commission is working with us to prevent or limit spread out of the Core Zone. 

If you are having work done on oak trees, such as pruning or felling, please check that your tree surgeon is familiar with the health and safety precautions they should follow, and with the regulations for disposing of oak material safely to avoid accidentally spreading the pest to new areas. These are available on the Forestry Commission website.

For detailed information about OPM, visit the Forestry Commission's website at Forestry commission - about the oak processionary moth.