Managing oak processionary moth in England

Oak processionary moth (OPM) (Thaumetopoea processionea)

If you own or manage oak trees, you should use this guidance to identify and survey the caterpillars and nests, and report any sightings.

You will also find information on government management of OPM and restrictions on the trade and movement of oak trees.

What is the OPM?

The caterpillars of OPM infest oak trees, eating (defoliating) the leaves, weakening the tree and leaving it vulnerable to other threats. OPM nests and caterpillars are also a hazard to human and animal health.

OPM was first identified in London in 2006 and has spread to some surrounding counties including Surrey and other parts of south east England. See the latest distribution map and check the OPM management zones map 2023 with exclusion zone (PDF, 1.36421 MBKB, 1 page) to see which management zone your trees are in.

OPM moths spread by flying from one tree to the next. Normally they stay close to the tree they were on as a caterpillar.

How to identify OPM

Caterpillars

The caterpillars have black heads and grey bodies covered in long white hairs. Please see these example OPM caterpillar images (PDF, 358 KB, 1 page).

The caterpillars are only about 2mm long when they emerge in Spring, and tend to remain high in the trees until they are older and larger. When they reach 1cm long they develop the irritating hairs. They’re fully grown at 2cm long.

They usually move nose-to-tail in a procession, hence their name. You may be able to see them on all parts of the tree - on the trunk, branches and leaves, and occaisonallyoccasionally on the ground.

Nests

Nests are usually found in early summer, in the trunks and branches of oak trees. A tree or branch can contain many nests. They can appear at any height - from the ground to the top of the tree.

You are unlikely to find nests on any other tree or shrub species, fences, walls or other similar structures (apart from structures supporting the tree).

They are typically dome or teardrop-shaped, and range from the size of a golf ball to a rugby ball. When fresh, the nests are white with white silken trails made by the caterpillars along the branches and leaves. But the nests soon become discoloured and brown, making them harder to spot.

Nests stay attached to the tree for many months after the adult moths have emerged.

When to look for OPM

Late spring and early summer are the best times to spot OPM caterpillars and their new nests in your oak trees. You can look for evidence of OPM from the ground, best checking from several angles. Using binoculars for this is recommended.

Spring and summer

The OPM caterpillars emerge from late March to May. They’ll move down the trees as they get older and bigger, and build nests.

Look for new nests in late spring and early summer when the caterpillars are active. Focus on the branches and trunks at all heights, and the ground for fallen nests later in the season.summer into autumn.

From late June to early August, the caterpillars retreat into the nests and form a pupa (cocoon). The pupae remain in the nests until they emerge as adult moths. 

Autumn and winter

Look for used nests in winter as they are easier to spot when there are no leaves on the oak tree or on the ground, but you can find them at any time of year.

Species commonly confused with OPM

We often receive reports of caterpillars which are not OPM, please take time to check this guidance Species commonly confused with oak processionary moth (OPM) (PDF, 5.17 MB, 1 page) for distinguishing between OPM caterpillars and those of other species before submitting any reported sighting.

How to report sightings of OPM and what happens after you file a report

Any sightings should be reported to the Forestry Commission via the Tree Alert online form. Alternatively, people can email opm@forestrycommission.gov.uk or call 0300 067 4442.

The Forestry Commission may issue you with a Statutory Plant Health Notice (SPHN) asking you to take action. This document will explain what action is needed.

In general, the Forestry Commission will take charge of treating infested trees in the buffer zone and pest free area. You can view these areas on the management zones map 2023 with exclusion zone (PDF, 1.36421 MBKB, 1 page). These areas comehave been updated and came into force on 18 March 2022.2023.

Removing nests and caterpillars is a hazardous operation. We therefore recommend that a professional does this work. You can find a professional on the Arboricultural Association website. They may spray the trees with an approved bio-pesticide or insecticide.

Risks of OPM

Do not touch or get close to the caterpillars or nests.

OPM caterpillars and nests affect the health of people and animals, as they contain hairs which can cause itchy rashes, eye and throat irritations and occasionally breathing difficulties in both people and animals.

In rare cases the hairs may cause an allergic reaction. You can become sensitised by repeated exposure to the hairs, worsening your symptoms.

People or animals can come into contact with OPM hairs if they touch OPM caterpillars and their nests by brushing past an infested oak tree or are near infested treestrees, as hairs can be blown about in the wind.

Animals can get hairs on their paws, and hairs can also get into their mouth and nose by sniffing, licking and picking up caterpillars or nests causing irritation. Livestock and horses can come into contact with hairs on the grass or other plants they eat, and by grazing close to a fallen nest.

The greatest risk is between May to July when the caterpillars are active. Although the risk is possible at any time of year due to residual hairs from old nests.

A public information leaflet covering the key facts, identification and control of OPM is available: Oak Processionary Moth - public information leaflet (PDF, 469 KB, 2 pages).

Reduce risk to people and animals

To avoid contact with the hairs, make sure you, and people or animals in your care, do not touch or go near nests or caterpillars.

Removing nests and caterpillars is a hazardous operation, it is recommended you call a suitably qualified arborist or pest control expert to remove infestations from your trees.

If you work on or close to oak trees in the affected areas, you need to take care and use protective measures, such as personal protective equipment (PPE). Read the guide on how to plan and manage risk of OPM for more information on controlling it and the PPE you need.

What to do if exposed

If you or someone in your care has a serious allergic reaction, get medical help. For less severe reactions, a pharmacist can provide advice on relief from skin or eye irritations.

If an animal in your care is seriously affected, get advice from a vet.

Tell the medical person or vet you suspect it is due to OPM contact.

You should report OPM via the Tree Alert online form, and let your neighbours who have oak trees know that they might also have an OPM infestation.

Further advice is available from the NHS website and the blue cross for pets.

Managing OPM

Forestry Commission leads an annual programme of work to manage OPM. The actions required under the programme will vary depending on where your site is located.

There are currently three areas or zones described below.

PestArea free areafrom the pest

This area is officially designated free from the pest and covers most of the country. In this area government funds an extensive programme of surveillance to monitor for OPM. If OPM is found in the pestarea free area,from the pest, government will take robust actions to eradicate it.

Buffer zone

In this area there is an annual OPM programme of surveillance and control, led by Forestry Commission. This programme involves detecting the pest and treating infested oak trees (and those trees in close proximity). If you own an infested oak tree in the buffer zone, the Forestry Commission may issue you with a Statutory Plant Health Notice (SPHN) asking you to take action. In most cases support will be available from the Forestry Commission for treatment of trees within this zone. The SPHN document will explain what action is needed.

Established area

This is the area where OPM is established in the South East of England. In this area landowners are responsible for the management of OPM on their land, however guidance is available on the OPM hub. If you own an infested oak tree in this area it is recommended you find a professional on the Arboricultural Association website to treat and remove any hazardous nest material if required.necessary. You should still check for OPM every year.

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If you own or manage oak trees, you should use the Oak Processionary Moth: plan and manage risks guidance to help plan and manage the risks from OPM.

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