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Find out about oak processionary moth

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Oak processionary moth (Thaumetopoea processionea; OPM) is a pest that has established itself in oak trees in recent years. It was accidentally introduced from mainland Europe.

The moth is a hazard to human and animal health. It can cause itching skin rashes and other health problems in people and animals.

OPM caterpillars can damage oak trees by feeding on the leaves. Large numbers can strip whole trees almost bare of leaves, but this is very rare.

Human and animal health

The caterpillars emerge about April every year. They develop thousands of tiny hairs which contain an irritating substance called thaumetopoein. This can cause itching skin rashes, eye irritations and sore throats. In rare cases they can cause breathing difficulties and allergic reactions.

The caterpillars shed hairs as a defence mechanism. The hairs are then carried by the wind. Caterpillar nests contain lots of hairs. They build nests on the trunks and branches of oak trees sometimes close to ground level. Nests can sometimes fall to the ground, and hairs can stick to the trunks and branches of oak trees.

OPM procession_H Kuppen
OPM cluster_Crown copyrightForestry Comission

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 nose-to-tail processions Their nests are white when new, and often have silken trails leading to them. They become discoloured and harder to see against the dark colour of oak tree bark. Caterpillars are most active April to July. Avoid nests, even empty nests at any time. This is because the hairs in them can remain irritating for many months.

How to report OPM

If you find OPM on our land, report this to parks and leisure services:

If you find OPM on other land:

Report OPM to the Forestry Commission (opens new window)

You will have to add a photograph to your report, but do not risk contact to get a photograph.
If you are unable to use the form you can contact the Forestry Commission on:

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

Note that OPM:

  • does not live on walls, fences or other structures
  • there is no need to report caterpillars or nests in these places

OPM can attack other trees, only if they run short of leaves to feed on. Only report caterpillars or nests in oak trees.

What to do if you are affected by OPM

The symptoms, although unpleasant, are not usually 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. Consult a vet if your animals have been in contact with the caterpillars or a nest.

If you own oak trees

The Forestry Commission will issue a Statutory Plant Health Notice if you have OPM in your oak trees and you live in the buffer zone. This notice requires you to have the pests removed. It does not mean that you are in any trouble - the notices are a management tool.

Guildford Borough lies in the OPM established area.

When having work done on oak trees, such as pruning or felling, check that your tree surgeon is familiar with OPM. They need to be aware of the health and safety precautions and disposal regulations. This is to avoid spreading the pests to new areas. These are available on the Forestry Commission website (opens new window).

For detailed information about OPM, visit the OPM resource hub (opens new window).