Oak processionary moth (Thaumetopoea processionea) adult Oak Processionary Caterpillars Oak processionary moth caterpilars wrapped around a twig White, silken nest of oak processionary moth (Thaumetopoea processionea) on trunk of oak tree Larvae of oak processionary moth (Thaumetopoea processionea) in classic nose-to-tail procession across the ground, London Skin rash caused by contact with hairs of oak processionary moth caterpillar Skin rash caused by contact with hairs of oak processionary moth caterpillar

Oak processionary moth (Thaumetopoea processionea)


The summer survey for oak processionary moth nests in oak trees in the London Control Zone  started on Monday 15th June, and will continue until about the end of July. Nests found are being carefully removed and destroyed.

The caterpillars are now large enough and descending low enough in the trees to be readily visible to the public. They have also by now developed the irritating hairs which cause itching skin rashes and other symptoms, so please observe the health precautions advised below, particularly if you work in or close to oak trees.

Managers of places with oak trees in or close to their properties should also by now have completed treatment of affected trees to protect themselves, the trees, and others who spend time at the sites. They should also now be surveying for nests and arranging to have these removed and destroyed. This includes private owners in the Core Zone. Our Oak Tree Owners' Manual and the maps in its Management Zones section  can help.

Meanwhile we have started erecting 850 pheromone traps around the outskirts of the known outbreak area to trap male adult moths as they emerge from pupation. Analysis of the results of the trapping programme will help us to update our knowledge of the distribution of the pest.

DescriptOak processionary moth (Thaumetopoea processionae) larvaion

Oak processionary moth (Thaumetopoea processionea, OPM) is a native of southern Europe, where local environmental factors and predators keep its populations in check. Aided by the trade in live plants, and possibly by the changing climate, it has become established as far north as the Netherlands and northern Germany. It was first accidentally introduced to Britain in 2005, and it is theoretically possible that if it were to spread it could survive and breed in much of England and Wales.

The larvae, or caterpillars, of OPM are a pest because they can be a hazard 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 on the trunks and branches of oak trees, and leave white, silken trails on the trunks and branches in early summer. These nests and trails 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

Like its close relative the pine processionary moth (PPM), the larvae move about in nose-to-tail processions. However:

  • OPM caterpillars often form a wedge-shaped procession, with one leader and subsequent rows containing several caterpillars;
  • PPM caterpillars are more likely to form a single nose-to-tail line.
  • OPM caterpillars are most likely to be found in oak trees;
  • PPM caterpillars are most likely to be found in pine trees.
  • OPM caterpillars are most likely to be seen in summer;
  • PPM caterpillars are most likely to be seen in winter and early spring.
  • OPM caterpillars have very long, white hairs which contrast markedly with other, shorter hairs;
  • PPM caterpillars are instead covered with dense clumps of hairs with less variation in length.

Caterpillars which can be mistaken for OPM

The threat

TO TREES: OPM caterpillars can threaten the health of several species of oak trees (Quercus species) 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 events such as drought and flood. They will attack other trees if they run short of oak leaves to eat, and have been observed attacking hornbeam, hazel, beech, sweet chestnut and birch trees.

TO PEOPLE AND ANIMALS: The caterpillars have thousands of tiny hairs which 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 into contact by the wind. The caterpillars can also shed the hairs as a defence mechanism.

Health precautions

In the affected areas, take some simple precautions to help minimise the health risks to you and your pets and livestock:


  • touch or approach nests or caterpillars;
  • let children touch or approach nests or caterpillars;
  • let animals touch or approach nests or caterpillars; and
  • try removing nests or caterpillars yourself.


  • teach children not to touch or approach the nests or caterpillars;
  • train or restrain pets from touching or approaching them;
  • see a pharmacist for relief from skin or eye irritations after possible OPM contact;
  • call NHS111 or see a doctor if you think you or someone you care for has had a serious allergic reaction;
  • 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; and
  • report sightings to your Council, or to the Forestry Commission using Tree Alert

Tree surgeons, forestry and ground-care workers and others working on or close to oak trees in the affected areas should wear full protective clothing, and familiarise themselves with the signs of OPM presence and the regulations applying to handling and moving oak material. This is available in the Good Practice Guidance document listed in the right-hand column of this page.

Further health advice is available from the NHS Choices website.

Outbreak stage

In the map below, the red dots indicate trees where OPM nests were found in London and Surrey during 2014 and the winter of 2014-15.

Map showing OPM Finds 2014 in the London area

Map of OPM nests, London, 2014-15 (PDF, 2.3MB)

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 and the Elmbridge and Spelthorne districts of Surrey (discovered 2006)

• Bromley and Croydon Boroughs in South London (2012)

• Pangbourne in West Berkshire (2010)

We believe it is not currently possible to eradicate the largest, West and South-West London/Surrey outbreak, but we and our partners are working hard to minimise its size, spread and impact as much as possible.

The Pangbourne outbreak is very small, with no nests found in 2013 or 2014, compared with 61 in 2011, indicating that it might be close to eradication. However, pheromone traps caught five adult male moths in late summer 2013 and three in late summer 2014, indicating that the species had not been eliminated from the area. We therefore continue to work with West Berkshire Council to eradicate the outbreak.

Evidence of OPM introductions was also found in Leeds in 2009 and Sheffield in 2010. However, annual follow-up surveys have found no evidence that these introductions have resulted in the establishment of breeding populations there. A single tree containing nests was found in East London in 2014, but follow-up surveys of oak trees in the surrounding area found no further evidence of nests or caterpillars in the area. The tree was destroyed before any adult moths could emerge from the nests and breed, and we continue to monitor the area.

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, although 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; and
  • 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 a Project Board comprising representatives of the Forestry Commission and Defra. The Project Board is supported by an Advisory Group comprising representatives of 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 new outbreaks of OPM follows our contingency plan.

Contingency plan

Most insecticide applications are undertaken from the ground. However, we treated about 10 hectares of woodland near Pangbourne in West Berkshire from the air, using a helicopter, in May 2013, and a small section of Sulham Wood, also near Pangbourne, which is public woodland which we manage, in 2014.

We undertook the aerial treatment in 2013 because OPM infestations were confirmed in the woods in 2012, when 'spent' nests were found. The adult moths which would have emerged from those nests were 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 treated part of Sulham Wood from the air because we perceived that adult male moths which were caught in pheromone traps nearby in late summer 2013 were most likely to have come from nests in Sulham Wood.

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, access and visibility in a woodland environment are usually impeded to the extent that effective treatment from the ground is impossible. Aerial spraying is therefore the only way to be reasonably sure of effective treatment in these circumstances.

Control programme proposals for 2015

Treatment and management

Do not 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 us via Tree Alert (below), or to your local council, and, if your trees are in the OPM Core Zone, 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.

Local councils

Our Plant Health Service

Larger land-managing organisations with professional grounds or tree-care staff might acquire their own equipment and train their own staff to do the job.

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

Oak tree owners' manual

Guidance for surveying and control

Survey and intervention guidance

Regulations, Restrictions and Powers

Among the key laws and regulations applying to OPM are amendments to the Plant Health (Forestry) Order 2005 (S12008/644) as it applies to England and Scotland, as follows:

• oak processionary moth shall not be introduced or spread within England or Scotland where this would threaten areas with 'protected zone status';

• all oak trees coming into the UK protected zone from another European Union Member State must be accompanied by an official statement, or 'plant passport', confirming that the plants are free from oak processionary moth; and

• all oak trees moving from the infested areas in London into the Protected Zone must be accompanied by an official statement, or 'plant passport', confirming that the plants are free from oak processionary moth.

There are no exceptions for these requirements: they apply to all forms of movement, including material intended for household use.

Protected Zone status: European Union legislation was introduced in October 2014 which recognises those parts of the UK that are outside the affected areas as a 'protected zone'. This legislation supersedes the previous national requirements, and means that all oak trees supplied to the protected zone must be free from the pest. The OPM Management Zones map in our Oak Tree Owners' Manual shows the boundary of the Protected Zone and the affected areas.

Notification of imports: The Plant Health (England) (Amendment) Order 2013, requires that pending landings of oak plants in England must be pre-notified to the plant health authorities. This Order also covered Platanus (Plane), Castanea (Sweet chestnut) and Fraxinus (Ash).  More details

Plant Health Notices: We may serve statutory Plant Health Notices on the owners of infested trees discovered in surveys, requiring them to remove the infestations. This is routine procedure which helps us with monitoring and management of the pest, 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.


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.

Symptoms and identification

Images of OPM

Identification guide

Reporting suspected cases

Please report any suspected finds to us, preferably using our Tree Alert form, or to your local council.

Before doing so please follow the health precautions above.

Please check:

  • that the affected tree is an oak tree. OPM caterpillars live almost exclusively in oak trees, and will generally only attack 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.
  • that 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.
  • that the nests are OPM nests. Many other, harmless moth and butterfly species build silken webbing nests in trees and shrubs, but OPM nests:

    • are usually roughly semi-spherical or teardrop-like in shape before they begin to collapse;
    • occur almost exclusively in oak trees;
    • are almost always attached to the trunks or branches of oak trees; and
    • are almost never woven among the leaves. Silken webbing nests among oak leaves, or in other trees and shrubs and other structures, are almost certainly NOT made by OPM and need not be reported .

Please report nests even if you do not see any caterpillars, because even spent nests can contain large numbers of the irritating hairs. Nests are also a useful sign that the pest is in the area. They will usually be found on the trunks or branches of 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 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, but can be as early as March. 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 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 of affected London Boroughs have been carried out each spring and summer since the West London outbreak was first discovered in 2006, and the larvae and nests found have been removed. Details of the action taken is in the reports, which are available on request.

Further information

For information about regulations and requirements applying to the import of oak plants, or to the movement, handling and disposal of oak material in the OPM-affected areas, contact our Plant Health Service on:

Tel: 0300 067 5155



Last updated: 07/02/2015