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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 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 (OPM) nests in oak trees in the London Control Zone , which started in June 2015, ended early in August. Nests found before the adult moths emerged were carefully removed and destroyed, as were nests found after emergence in high-traffic areas such as public parks and gardens, because of the risk to public health which they pose from the detached caterpillars hairs left in them.

The main focus of activity is now on the pheromone trapping programme. We and partners, including the Woodland Trust, have about 900 pheromone traps deployed around the outskirts of the known outbreak area to trap male adult moths as they emerge from pupation. These traps are being checked regularly for moth captures from early August into September. Analysis of the results of the trapping programme will help us to update our knowledge of the distribution of the pest.

Some limited surveying for nests continues in areas where OPM had not been found previously as we work to ascertain as accurately as possible the extent of local distribution.


Oak processionary moth (Thaumetopoea processionae) larva

Oak processionary moth (Thaumetopoea processionea, OPM) 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 is a native of southern 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 spring-time larval emergence period in recent years might have enabled it to become established further north than would have been possible in the past.


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 (never among the leaves), 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

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Key identifying features are that the larvae (caterpillars):

  • move about in nose-to-tail processions;
  • often form arrow-headed processions, with one leader and subsequent rows containing several caterpillars abreast;

  • are most likely to be found in oak trees;

  • are most likely to be seen in summer;

  • have very long, white hairs which contrast markedly with other, shorter hairs; and
  • do not live on fences, walls and similar structures, as some caterpillar species do.

To help you distinguish OPM caterpillars from those of other species see

Caterpillars easily mistaken for OPM and this information about the

Pine processionary moth (PPM).

However, please note that there are no known populations of pine processionary moth in Great Britain.

The threat

OPM caterpillars can threaten the health of several species of oak trees (Quercus species) because they feed on the leaves. Large populations can defoliate, or strip bare, large parts of oak trees, 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 feed on other trees if they run short of oak leaves to eat, and have been observed on 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
  • 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
  • keep horses and livestock a safe distance from infested oak trees.
  • 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
  • Report any sightings to us or your local council. (See Reporting Sightings below.)

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.

See our Good Practice Guide on handling oak material

Further health advice: NHS Choices

Outbreak stage

Surveys of affected London Boroughs and, more recently, northern districts of Surrey, 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.

On the map below, the red dots indicate where OPM nests have been found in 2015 in London and Surrey. The green-dotted line indicates the boundaries of the known outbreak areas in 2014. Data for 2015 is not yet available for all nests found in the pink-shaded Core Zone.

OPM infestation survey results map

Map - OPM nests, London

There are three confirmed outbreaks of breeding OPM in Britain, all 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); and

• 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, and none so far in 2015, indicating that it might be close to eradication. However, pheromone traps have caught small numbers of adult moths in each of 2013, 2014 and 2015, indicating that the species has not yet 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. A single tree containing nests was found in East London in 2014, and it was destroyed before any adult moths could emerge from the nests and breed. Follow-up surveys of oak trees in the surrounding area immediately afterwards found no further evidence of nests or caterpillars in the area, but several nests were found in nearby oak trees in 2015. These were immediately destroyed, and monitoring continues.

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


Control action includes:

  • winter surveying for old 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, making this option practicable only on very small trees;
  • 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 - the most reliable and effective method of control - 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 with  Forestry Commission and Defra representatives. The 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

Control programme proposals for 2015

Treatment and management

To be most effective the job should be carefully timed and carried out by professionals with appropriate training and equipment. Do not try to remove OPM caterpillars or nests yourself, because of the health risks. Report the presence of the pest 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.

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.

Our Oak tree owners' manual provides comprehensive advice on all aspects of OPM management.

Most insecticide applications are undertaken from the ground. However, we treated two small areas of woodland near Pangbourne from the air in 2013 and 2014. This is because OPM is very difficult to treat from the ground in a closed-canopy woodland environment, and because early in the caterpillar stage, when insecticide treatment is most effective, they live very high in the trees.

Guidance for surveying and control

Guidance on survey and intervention for different phases of the species' life cycle.

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

• 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 detail

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.

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

Reporting suspected cases

Please report any suspected finds to us, but before doing so please check the identification section above to:

  • ensure 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.;
  • ensure 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; and
  • ensure that the nests are OPM nests. Many other, harmless caterpillars build silken webbing nests in trees and shrubs, but OPM nests:

    • are usually roughly semi-spherical, teardrop-like in shape before they begin to collapse, when they can become bag-like;
    • occur almost exclusively on 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 on 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 fall to the ground.

We do not need reports of adult moths, which are difficult to identify accurately.

Once you are satisfied that you have seen OPM nests or caterpillars, and not those of another species, report it to us:

• by email to (preferred); or

• by phone to 0300 067 4442.

When reporting sightings, please be sure to:

  • Provide a precise location of the tree/s.

o A 10-digit Ordnance Survey grid reference is ideal;

o otherwise provide a full address, including property name and/or street or road number and full postcode; and/or

o precise instructions for finding the tree/s, e.g. “35 metres north-west of the park entrance in XXX Street”;

  • provide a telephone number where we can reach you during the daytime to clarify any points; and
  • provide a clear, well lit photograph if you can (but please do not risk contact with OPM hairs to get a photograph).

If you prefer you can contact your local council:

Council OPM contacts

Further information

For information about regulations and requirements applying to the importation 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: 09/08/2015