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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 larvae, or caterpillars, of the oak processionary moth (OPM; scientific name Thaumetopoea processionea) can affect the health of oak trees, people and animals. They feed on oak leaves, and large populations can strip trees bare, leaving them weakened and vulnerable to other threats. A protein in the caterpillars' tiny hairs can cause skin and eye irritations, sore throats and breathing difficulties in people and animals who come into contact with them.

The species derives its common name from the fact that the caterpillars live primarily on oak trees and move about in nose-to-tail processions; it derives the first part of its scientific name from thaumetopoein, the irritating protein in the caterpillars' hairs.

In the United Kingdom, OPM is known to be present only in London and some neighbouring counties. It is subject to a government-led programme of survey and management in these areas to minimise populations, spread and impacts. A breeding population was also discovered in Pangbourne, West Berkshire in 2010, but following eradication action no nests or caterpillars have been seen there since 2012. 


OPM caterpillars were being spotted in situations where they are visible and recognisable to the public in the week beginning 14th May. This means they are now also developing the hairs which contain the irritating protein, so we advise people to follow the precautions given below to protect themselves and their animals from exposure.

Caterpillars were spotted emerging from egg plaques in mid-April, so we began treating trees in the Protected Zone with an approved biopesticide during the week beginning Monday 23 April 2018. The treatment programme is expected to continue until late May or early June. After that the caterpillars will be too large to be affected by our preferred treatment product.

The oak trees being treated are those known to have been infested with OPM last year and in 2016, and other oak trees nearby. We will treat oak trees at more than 600 sites in the 'Control Zone' in and around London, and local authorities and other tree managers are likely to treat more in the Core Zone.

A summer survey for nests in the Control and Protected Zones will follow. Oak tree owners in the Core Zone are strongly encouraged to do the same.

The caterpillars will complete pupation into adult moths in their nests on the trunks and branches of oak trees by early September. During this period we will once again undertake pheromone trapping of male moths, which, although it cannot make a significant impact on the pest's population, can alert us to possible changes in the its distribution. We welcome offers of sites for the pheromone trapping programme, and any landowner or manager who can offer sites, especially around the periphery of the known affected areas, is invited to get in touch with us. We can provide traps.

The map at the link under "Outbreak stage" below indicates the locations of nest findings since the Spring of 2017. Meanwhile, a report on the year's operation has been published, and earlier years' programme reports are available on request.

We have issued the first two 'Programme Update' newsletters of the season, and the latest one is available from the right-hand column of this page. 

Health precautions - advice on avoiding contact with the caterpillars

Reporting suspected cases 

OPM manual - help and advice for owners of affected oak trees, and detailed explanation of the management regimes applying in the Core, Control and Protected Zones.


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 the movement of live oak plants in trade, its range has been expanding northwards over the past 20 years, and it has become established as far north as the Netherlands and northern Germany. 

OPM was first accidentally introduced to Britain in 2005, almost certainly as eggs which had been laid on live oak plants imported from continental Europe. The current distribution of the pest has probably arisen from a number of subsequent, similar introductions as well as spread from the original points of introduction. It is theoretically possible that if it were to spread it could survive and breed in much of England and Wales.


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 live and feed almost exclusively on oak trees. They can sometimes be seen processing across the ground between oak trees, and clustering together as they feed on oak leaves.

Our Tree Name Trail page on oak trees can help you to identify oak trees.

In early summer they build distinctive white, silken webbing nests on the trunks and branches of oak trees (almost never among the leaves), and leave white, silken trails on the trunks and branches. These nests and trails become discoloured after a short time, and more difficult to see as a result.

  • NOTE: OPM nests are almost never made among the leaves of oak trees or any other tree or shrub, or on any structure. Such nests are usually made by harmless species, and need not be reported.

The nests can occur in a range of shapes, including hemispherical (half a ball), tear-drop shaped, bag-like, and like a blanket stretched around part of a trunk or branch. Sizes range from as small as the width of a 50p coin to stretching several feet up the oak tree trunk in some cases. They can occur anywhere from ground level to high in the oak tree, and can fall out of oak trees and be found on the ground.

The caterpillars rest up in these nests during the day between feeding periods, and later in the summer they retreat into the nests to pupate into adult moths.

The adult moths emerge from pupation and are active from mid to late summer, and lay their eggs on the small twigs and branches in oak trees. They are an undistinctive, brown moth very similar in appearance to other, harmless species, and because of the difficulty in accurately identifying them, we do not require reports of moth sightings.



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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, and sometimes on the ground under oak trees;
  • are most likely to be seen in late Spring and early 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 distinguish OPM caterpillars from those of other species see:

Caterpillars easily mistaken for OPM 

Pine processionary moth (PPM).

(There are no known populations in the UK of pine processionary moth (Thaumetopoea pityocampa), a closely related species which lives in pine trees.)

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 defoliate, or strip bare, large parts of oak trees, leaving them vulnerable to attack by other pests and diseases, and less able to withstand stresses such as drought and flood. They will only feed on other trees if they run short of oak leaves to eat, and have been seen on hornbeam, hazel, beech, sweet chestnut and birch trees.

To people and animals: The caterpillars' thousands of tiny hairs which contain an urticating, or irritating, substance called thaumetopoein. 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, and lots of hairs are left in the nests, which is why nests should not be touched without protective clothing.

Health precautions

People in the affected areas can take these simple precautions to minimise the health risks to themselves and their 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; or
  • 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. Covering or stabling livestock can help;
  • see a pharmacist for relief from skin or eye irritations after suspected OPM contact;
  • call NHS111 or see a doctor if you think you or someone in your care has had a serious allergic reaction;
  • consult 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

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.

Good Practice Guide on handling oak material

OPM Manual -  Occupational health advice

NHS Choices - further health advice

Outbreak stage

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

In the map at the link below:

  • the red dots indicate where OPM nests were found in 2017-18;
  • the mauve dots indicate where nests were found in 2016-17 and reported to us;
  • the blue triangles indicate where trees were surveyed for OPM in 2017-18, but no nests were found. (In some cases, but not all, these trees were infested in previous years.)
  • the orange line indicates the boundaries of the 2018 Core Zone; and
  • the green dotted line indicates the outer boundary of the known affected area in 2016-17.

Map - OPM nests, London

The confirmed outbreaks of breeding OPM in Britain are all in southern England, as follows:

  • several boroughs in West, South-West and South London, the Elmbridge, Spelthorne and Runnymede Districts of Surrey, and the Slough and South Buckinghamshire Districts of Buckinghamshire, first discovered in 2006;
  • the Pangbourne area of West Berkshire, 2010;
  • several London Boroughs in East and North-East London and Epping Forest District in Essex, 2014, (although no nests were found in Epping Forest District in 2017);
  • Guildford and Mole Valley Districts in Surrey, 2015;
  • Watford Borough in Hertfordshire, 2016, (although no nests were found in 2017); and
  • Thurrock District of Essex, 2017.

The outbreak in the Pangbourne area of West Berkshire was very small, and no nests have been found since 2012. However, pheromone traps caught small and diminishing numbers of adult male moths in each of the past five years: only trwo were trapped in 2017. This suggests that it is unlikely that a sustainable population remains in the area. We have therefore decided not to survey for nests or monitor for male moths with pheromone traps in Pangbourne in 2018 because we are unlikely to obtain any useful information. However, this can be reviewed if evidence emerges that the situation might be changing.

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 resulted in the establishment of breeding populations.

Most of the 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 previously been laid. However, we do not have any clear evidence for the how the insect got to Guildford, Mole Valley and Watford, and we will not know how it got to Thurrock without further investgation. The Thurrock population was found late in the season as a result of pheromone trapping evidence.

OPM Manual - more details on OPM management zones


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. However, 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. 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 help us to monitor changes in the distributon of the pest.

Operations are planned and co-ordinated by three boards - project, evidence and operational - comprising Forestry Commission and Defra representatives. These boards are advised by entomologists from our Forest Research agency. Our response to new outbreaks of OPM follows our contingency plan.

Contingency plan

2017-18 Operational Plan

Treatment and management

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

Good Practice Guidance

Oak tree owners' manual - 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; 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. 

Our Oak Tree Owners' Manual shows the boundary of the Protected Zone and the affected areas.

Notification of imports: The Plant Health (England) Order 2015, requires that pending landings of oak plants in England must be pre-notified to the plant health authorities. This order also requires pre-notification of other tree species. more

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. This is also when they develop the irritating hairs which pose the health problems.

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 and cast-off hairs, and should not be handled without personal protecive clothing.


OPM Pest Risk Analysis 

Scientific research


A range of tools designed to help manage OPM and communicate the issues.

Reporting suspected cases

Tree Alert icon Before you report a suspected sighting, please check our identification section.


If you cannot use Tree Alert:

But please use Tree Alert if you can. Note that it requires a photograph to be uploaded.

Before reporting a suspected sighting, 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 moth 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;
  • 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 or teardrop-like in shape before they begin to sag and 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 among the leaves of other trees and shrubs, and nests on other structures such as gates, fences and walls, 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 a large number 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.

If reporting sightings by email or phone, please be sure to:

  • Provide a precise location of the tree/s.

- A 10-digit Ordnance Survey grid reference is ideal

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

- 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 with email reports if you can (but please do not risk contact with OPM hairs to get a photograph).

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 Cross-Border Plant Health Service.

T: 0300 067 5155



Last updated: 16th May 2018