Oak processionary moth (Thaumetopoea processionea)

The threat
Health precautions
Outbreak stage and map
Managing affected trees
Survey and control
Regulations and powers
Reporting suspected cases
Import restrictions
Further information


DescriptOak processionary moth (Thaumetopoea processionae) larvaion

There are outbreaks of Oak processionary moth (Thaumetopoea processionea, OPM)  in West and South-West London, the Bromley/Croydon area of South London, and the Pangbourne area of Berkshire.

The larvae, or caterpillars, of OPM are a pest because they pose a threat to oak trees and to human and animal health. 

OPM caterpillars are most easily recognised by their distinctive habit of moving about in late spring and early summer in nose-to-tail processions, from which they derive their name, and the fact that they almost exclusively live in and feed on oak trees.

They also build distinctive white, silken webbing nests in oak trees and leave white, silken trails on the trunks and branches in early summer. These become discoloured after a short time, and more difficult to see as a result.

The adult moth is an undistinctive, brown moth very similar in appearance to other, harmless species. They are active from mid to late summer and lay their eggs on the smaller twigs and branches in oak trees.



Identification guide

Youtube videos

Caterpillars which can be mistaken for OPM

The threat

OPM caterpillars can threaten the health of oak trees because they feed on the leaves. Large populations can strip whole oak trees bare of leaves, leaving them vulnerable to attack by other pests and diseases, and less able to withstand adverse environmental factors such as drought and flood.

The caterpillars have thousands of tiny hairs contain an urticating, or irritating, substance called thaumetopoein, from which the species derives part of its scientific name. Contact with the hairs can cause itching skin rashes and, less commonly, sore throats, breathing difficulties and eye problems. This can happen if people or animals touch the caterpillars or their nests, or if the hairs are blown about by the wind.

Health precautions

This pest is a danger to oak trees, to human health and to animal health.

Some simple actions can help keep the risk of serious harm or disruption very low.


• touch or approach nests or caterpillars

• let children touch or approach nests or caterpillars

• let animals touch or approach nests or caterpillars

• try removing nests or caterpillars yourself – call an expert


• teach children not to touch or approach the nests or caterpillars

• seek medical advice if you think you or someone you care for has been seriously affected

• see a vet if you think your pet or livestock has been seriously affected

• call in a pest control expert to remove infestations in your own trees

• report sightings of OPM to your Council, or to the Forestry Commission using Tree Alert (see below)

• train or restrain your pets from touching or approaching them.

Advice on the symptoms and their treatment is available from the Public Health England website.

Outbreak stage

Map of London boroughs with OPM outbreaks

Map: Orange shading shows areas of London outbreaks in 2013.

Large sized map

There are three confirmed outbreaks of breeding OPM in Great Britain, all of them in southern England:

• several boroughs in West and South-West London (discovered 2006)

• Bromley and Croydon Boroughs in South London (2012)

• Pangbourne in West Berkshire (2010)

It is no longer possible to eradicate the West and South-West London outbreak, but we and our partners are working hard to minimise its population, spread and impact as much as possible.

The Pangbourne outbreak is very small, with no nests found in 2013, compared with 61 in 2011. However, pheromone traps caught five adult male moths in 2013. We are working with West Berkshire Council to eradicate theoutbreak. We are also working with partners to attempt to eradicate the South London outbreak, which is still relatively small and in a concentrated area.

Evidence of OPM introductions was also found in Leeds in 2009 and Sheffield in 2010. However, follow-up annual surveys have found no evidence that these introductions have resulted in the establishment of breeding populations there.

All of these introductions and outbreaks are thought to have resulted from the importation from nurseries in continental Europe of young oak trees on which over-wintering OPM eggs had been laid.


Action to control OPM includes:

• winter surveying for spent nests in oak trees in affected and at-risk areas, which can tell us where we might expect to find the pest the following spring and summer

• destruction of egg masses before the eggs hatch the following spring. However, egg masses are very difficult to find

• surveying of oak trees for signs of eggs, caterpillars, nests and other evidence during the spring and early summer, and marking these for treatment

• carefully controlled treatment of affected trees with approved insecticide in spring to kill the caterpillars soon after they emerge. This is the most reliable and effective method of control, and is where we have concentrated our efforts

• manual removal of nests and caterpillars by suitably trained and equipped operators, usually using vacuum equipment, during the brief pupal stage, thereby reducing the number of adult moths which will emerge from the pupae.

• pheromone trapping of adult male moths in late summer and early autumn, which can tell us where we might expect to find the pest the following year.

Operations are planned and co-ordinated by Outbreak Management Teams or liaison groups comprising the Forestry Commission, local authorities, major land-owning and land-managing organisations, Public Health England and regulatory authorities. These groups are advised by entomologists from the Forestry Commission’s Forest Research agency. Our response to outbreaks of OPM follows our contingency plan. 

Contigency plan

We treated about 10 hectares of woodland near Pangbourne in West Berkshire from the air, using a helicopter, in May 2013.

We sprayed these woods because OPM infestations were confirmed in them last year when 'spent' nests were found. The adult moths which would have emerged from those nests are highly likely to have mated and established a breeding population in the woods, raising the prospect of the pest spreading into other areas and creating an ever-increasing problem.

We opted for aerial spraying rather than ground-based treatment because OPM is very difficult to find in a closed-canopy woodland environment, and it can be difficult to treat effectively from the ground, largely because early in the caterpillar stage they live very high in the trees. Ground-based treatment, including spraying and manual removal, requires good visibility and unrestricted access to the trees. Although these techniques are routinely used in parks and gardens, in a woodland environment access and visibility are usually impeded to the extent that treatment from the ground is impossible. Aerial spraying is therefore the only way to be reasonably sure of effective treatment in these circumstances.

Managing affected trees

Don't try to remove OPM caterpillars or nests yourself because of the health risks, and because to be most effective the job should be carefully timed and carried out by professionals with appropriate training and equipment. Report the presence of the pest to your local council or to our Plant Health Service and get a professional pest control operator to remove the infestations. Your local council or our Plant Health Service can provide details of suitable pest control operators working in the area.


Plant Health Service

Larger land-management organisations with professional grounds or tree-care staff might consider acquiring their own equipment and training their own staff to do the job.

Even if OPM is not known to be present in local trees, tree surgeons and others working on or close to oak trees in affected areas are strongly advised to wear protective clothing.

Guidance for surveying and control

Surveying, larvae treatment and nest removal guidance

Survey and intervention guidance

Regulations and Powers

Among the key laws and regulations applying to OPM is an amendment to the Plant Health (Forestry) Order 2005 (S12008/644) requiring that:

• all oak plants coming into the UK from another European Union Member State must have a plant passport  confirming that the plants have been grown in a nursery where the moth is not present  and which has been free of symptoms for at least the most-recent growing season

• imported oak plants must be a maximum 2 metres in height, (based on evidence that OPM are unlikely to lay eggs in trees smaller than this)

• material from felled or pruned oak trees in the affected areas may not be moved outside the areas without consulting the Forestry Commission’s Plant Health Service first for advice about handling and moving the oak material. (The UK outside the affected areas is an EU Protected Zone for OPM.)

The Plant Health (England) (Amendment) Order 2013, which came into force on 17 January 2013, requires that pending landings of oak plants in England must be pre-notified to the plant health authorities. Our Plant Health Service can provide full details of the regulations.

We may serve statutory Plant Health Notices on the owners of infested trees discovered in surveys, requiring them to remove the infestations. Issuing these notices is routine procedure, and does not mean that owners are in any trouble. However, failure to comply with a notice can result in enforcement action and possible prosecution.

Local authorities can also issue notices requiring removal of infestations for environmental or public health reasons.


OPM is a native of southern and central Europe, where predators and environmental factors usually keep its numbers in check and minimise its impact. However, aided by movement of plants, its range has been expanding northwards over the past 20 years, and it has become established as far north as the Netherlands, Belgium and northern Germany. Climate change might be a factor - the reduced incidence of cold, wet weather during the springtime larval emergence period in recent years might have enabled it to become established further north than would have been possible in the past.

Reporting suspected cases

Please report any suspected finds of Oak processionary moth to us using our Tree Alert app or on-line form below, or to your local council.

However, before doing so, please check that:

  • The affected tree is an oak tree. OPM caterpillars live almost exclusively in oak trees, and will generally only occupy other trees if they become very short of oak leaves to eat. Oak trees are fairly easy to identify by their distinctive leaves and bark. OPM caterpillars do not live on fences and similar structures, as some caterpillar species do.
  • The caterpillars are oak processionary caterpillars. They have distinctive habits of moving about in nose-to-tail processions and clustering together. They are also most likely to be seen in or near oak trees, and even when seen on the ground, this will usually be close to oak trees. Identification guidance.
  • The nests are OPM nests. Many moth and butterfly species build silken webbing nests in trees and bushes. OPM nests are usually roughly semi-spherical in shape. They occur almost exclusively in oak trees; are almost always attached to the trunks or branches; and are not woven among the leaves, as some other species' nests are.

Please report nests even if you do not see any caterpillars with them, because even spent nests can contain thousands of the irritating hairs. Nests are also a useful sign that the pest is in the area. They will usually be found in oak trees, but they can be dislodged and might be on the ground.

We do not need reports of adult moths.

Tree Alert iconIf you think you have spotted the pest, please check the symptoms section above before using our Tree Alert form:


 or our free App.

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or by contacting your local Council .

Life cycle

Adult moths emerge from the pupae in mid to late summer. They live for an average of three days, during which time they mate and lay their eggs in healthy oak trees, usually high in the canopy.

The eggs spend the winter on the trees, and the larvae/caterpillars emerge the following spring. Larval emergence begins about mid to late April in an average spring. As they grow, the caterpillars descend lower in the trees to feed and build nests, and this is when they are most likely to be seen by the public. It is also when they develop the irritating hairs which pose the health problems. They spend their days in the nests, emerging mostly at dawn and dusk to feed, although this pattern can vary. They can sometimes be seen processing across the ground between oak trees.

In late June and July the caterpillars congregate in the nest to pupate into adult moths, which can take up to four weeks. Spent nests found after adult moth emergence will invariably contain pupae cases.


OPM Pest Risk Analysis 

Scientific research Surveys

Surveys of the affected London Boroughs have been carried out each spring and summer since the outbreak was first discovered in 2006, and the larvae and nests found have been removed. Details of the action taken are contained in the reports which are available on request.

Import restrictions

Following the findings of the 2007 survey a formal request was made to the European Commission to consider designating Britain as an EU 'protected zone' with measures to prevent further accidental introductions, which we considered necessary for eradication attempts to succeed.  Pending a final Decision, we put in place temporary emergency measures by amending the Plant Health (Forestry) Order 2005 (SI2008/644).  This came into force on 31 March 2008, and requires that all oak trees coming from another Member State must have a plant passport confirming that the plants have been grown in a nursery where the moth is not present and which has been free of symptoms for the last complete growing season.

New requirements for statutory notification of imports of Quercus (Oak) as well as Platanus (Plane), Castanea (Sweet chestnut) and Fraxinus (Ash) - came into effect on 17 January 2013.

New requirements (Fera)

Further information

0131 314 6414


Last updated: 04/15/2014