The caterpillars (larvae) of oak processionary moth (OPM) are pests of oak trees, and a hazard to human and animal health. OPM was first accidentally introduced to England in 2005, and is subject to a government-led programme of survey and control to minimise its population, spread and impacts.
OPM is established in most of Greater London and in some surrounding counties. See the distribution map for its current known distribution, as indicated by finds of their distinctive silken nests. In the map:
the red dots indicate OPM nests found in 2018. (Not all nests found in the Core Zone are recorded);
the blue triangles indicate where trees were surveyed for OPM in 2018, 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 2017.
A breeding population was also discovered in Pangbourne, West Berkshire in 2010, but following eradication action, no nests or caterpillars have been found there since 2012. However, very small numbers of male moths have been trapped in the area every year since.
The threat to trees
OPM is a tree pest because its caterpillars feed on the leaves of several species of oak trees. Large populations can strip whole oak trees bare, leaving them more vulnerable to other pests and diseases, and to other stresses, such as drought. The picture above shows branches of a London oak tree after heavy feeding by OPM caterpillars.
The threat to people and animals
Older caterpillars develop tiny hairs containing an urticating, or irritating, protein called thaumetopoein. Contact with the hairs can cause itching skin rashes (pictured above) and eye irritations, sore throats, breathing difficulties and, rarely, allergic reactions in people and animals. The risk of exposure to these hairs is highest in May and June.
The caterpillars can shed the hairs when threatened or disturbed. The hairs can be blown by the wind, and they accumulate in the caterpillars’ nests, which can fall to the ground. They can stick to trunks, branches, grass and clothing as well as tree surgeons’, forestry and ground-care workers’ equipment, such as ropes.
Among the groups most vulnerable to the health hazards are:
- curious children and pets;
- people who work on or close to oak trees;
- anyone spending time close to infested trees; and
- grazing and browsing livestock and wild animals.
OPM moths, the adult form of the species, are undistinctive brown moths similar to other species, and are difficult to accurately identify. They are not a health hazard, and we do not require reports of sightings.
- have a distinctive habit of moving about in late spring and early summer in nose-to-tail processions, from which they derive their name. The processions are often arrow-headed, with one leader and subsequent rows of several caterpillars abreast;
- live and feed almost exclusively on oak trees. They can sometimes be seen processing across the ground between oak trees;
- will usually only affect other broad-leaved tree species if they run short of oak leaves to eat – they have been observed feeding on sweet chestnut, hazel, beech, birch and hornbeam. However, they generally cannot complete their development on other tree species;
- cluster together while they are feeding on oak leaves and moving from place to place;
- are only seen in mid- to late spring and early summer (May, June and July);
- have very long, white hairs which contrast markedly with the much shorter, almost undetectable irritating hairs;
- have a grey body and dark head. Older larvae have a central dark stripe with paler lines down each side; and
- are not usually found on fences, walls and similar structures, such as garden furniture.
To help distinguish OPM caterpillars from those of other species, see:
- Caterpillars easily mistaken for OPM; and
- Pine processionary moth (PPM). There are no known UK populations of pine processionary moth (Thaumetopoea pityocampa), which is a closely related species which lives on pine trees.
- are built in early summer;
- are made on the trunks and branches (pictured) of oak trees;
- are almost never made among the leaves of oak trees, or on any other tree or shrub species, or on fences, walls and similar structures. Such nests are usually made by harmless insect species, and need not be reported;
- are made of distinctive, white, silken webbing, which are accompanied by white, silken trails on the trunks and branches of oak trees;
- become discoloured after a short time, and more difficult to see as a result, as do the silken trails;
- occur in a range of shapes, including hemispherical (half a ball), tear-drop shaped, bag-like, and like a blanket stretched around part of an oak trunk or branch;
- range in size from a few centimetres wide to stretching several feet up the trunk;
- can occur anywhere from ground level to high in the oak tree;
- can fall out of oak trees and be found on the ground; and
- can remain attached to the trees for many months after the larvae have pupated and the adult moths have emerged.
Several nests can occur on the same tree or branch.
Older caterpillars feed mainly at night and rest up in their nests during the day. Later in the summer they retreat completely into the nests as pupae, re-emerging a few weeks later as adult moths.
‘Spot it, avoid it, report it’ is the mantra we encourage everyone in affected areas to adopt. In practice this means that everyone should learn how to:
- recognise OPM nests and caterpillars;
- protect themselves and the people and animals in their care from the health hazards – see ‘Health precautions’ below; and
- report a finding – see ‘Report a sighting’ below.
Owners and managers of oak trees in the affected areas should also familiarise themselves with the ways in which they can:
- fulfil their duty of care to other people and animals;
- comply with the regulations governing OPM control and the movement of oak plants; and
- comply with the regulations governing the handling and movement of oak material arising from felling, forestry operations and tree surgery.
To minimise the health risks:
- do not touch or approach OPM nests or caterpillars;
- do not let children or animals touch or approach nests or caterpillars; and
- do not 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 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 – tell the doctor you suspect OPM contact;
- consult a veterinary surgeon if you think your pet or livestock has been seriously affected – tell the vet you suspect OPM contact;
- call in an arborist or pest control expert with relevant expertise to remove infestations in your own trees. Your local council, the Arboricultural Association, the British Pest Control Association or the Forestry Commission can provide a list of suitable operators in your area; and
- if you work on or near oak trees in the affected areas, for example, as a tree surgeon or forestry, landscaping or ground-care worker, wear full protective clothing. The Oak Tree Owners’ OPM Manual has occupational health guidance.
Report a sighting
If you believe you have found an OPM nest or caterpillar, please report it immediately to us.
Use our Tree Alert on-line pest and disease reporting form to report sightings.
Your reports will be assessed by our scientists and forwarded to the Forestry Commission for the appropriate action.
You will need to upload a clear, well lit photograph to Tree Alert, but do not risk contact with the hairs to get it.
Before reporting a suspected sighting, please check the ‘Identification’ section above and the pictures in our OPM Manual to ensure that:
- the affected tree is an oak tree. Most oak trees are fairly easy to identify by their distinctive leaves and bark; and
- the caterpillars are oak processionary moth caterpillars and not those of another species.
Report nests even if you do not see any caterpillars, because nests are a useful sign that the pest is in the area. Do not touch ‘spent’ nests, which can contain large numbers of the irritating hairs.
Do not report adult moths, which are difficult to accurately identify, and are monitored by pheromone trapping. However, we do accept reports of OPM moths, especially females, caught in light traps by moth recorders who can accurately identify them.
If you cannot use Tree Alert, you may report a sighting by:
- email to email@example.com; or
- telephone to 0300 067 4442.
If reporting sightings by email or phone, please include:
- a precise location of the tree/s – a 10-digit Ordnance Survey grid reference is ideal, e.g AB 12345 67890. Otherwise provide a full address, including property name and/or street or road number and the full postcode; and/or
- precise instructions for finding the tree/s, e.g. “40 metres north-west of the entrance to (name) Park in (name) Street”;
- a telephone number where we can reach you during the daytime to clarify any points;
- a clear, well lit photograph with email reports if you can; and
- contact details of the owner or manager of the tree/s, if known.
Treatment and nest removal
To be most effective, tasks such as insecticide spraying (pictured) and nest removal should be carefully timed and carried out by professionals with appropriate training and equipment. Owners and managers of larger land-holdings with professional ground-care or tree-care staff might acquire their own equipment and train their staff to do this work. Our Oak Tree Owners’ OPM Manual has guidance on:
Government action, and government support for affected owners, depends on which one of the three OPM management zones the affected trees are in. They are:
- the Core (central) Zone of the affected area;
- the Control, or buffer, Zone around the Core Zone; and
- the Protected Zone, which comprises the remainder of the UK.
The Oak Tree Owners’ OPM Manual includes guidance on the zones.
The Defra-funded, Forestry Commission-led programme of OPM survey and control operates in the Control and Protected Zones. It is aimed at preventing the pest from spreading, and at reducing its impacts. The programme includes:
- surveying oaks in spring and summer for signs of caterpillars, nests and other evidence, such as silk trails;
- carefully controlled treatment of affected trees with an approved insecticide or bio-pesticide in spring to kill the caterpillars soon after they emerge. This tends to take place about mid-April, with some flexibility each side. This is the most reliable and effective method of control, and is where the Forestry Commission’s government-funded efforts have been concentrated; and
- pheromone trapping of adult male moths in late summer, which can help us to detect changes in the distribution of the pest, such as spread into new areas.
Manual removal of nests in all three zones is at the discretion of the owners of affected oak trees, although we strongly encourage it. It is most effectively done by suitably trained and equipped operators, wearing the appropriate personal protective clothing. These operators will remove the nests using vacuum equipment or by hand, and will bag the nests safely for disposal and incineration.
Nest removal is best carried out after the caterpillars have pupated, when all of the pupae are inside the nests, because this reduces the number of adult moths which will emerge to lay eggs. It also reduces the local health hazard. Once the adult moths have emerged, nest removal no longer provides a means of control, but it can still be useful in reducing the health risk from falling nests and the caterpillars’ hairs.
Survey and control in the Core Zone is the responsibility of oak tree owners. However, the Forestry Commission reserves the right to issue Statutory Plant Health Notices (SPHNs) requiring infestation removal in the Core Zone if severe infestations threaten to spread into the Control Zone. Local councils may also use their public health and safety powers to require removal.
Forestry Commission operations funded by Defra are planned and co-ordinated by three boards – project, evidence and operational. These boards are advised by entomologists from Forest Research.
The Forestry Commission’s response to new outbreaks follows its OPM contingency plan.
OPM is subject to regulations intended to minimise the risk of further introductions to the UK and spread to new areas. Forestry workers, tree surgeons, landscapers, nursery staff and anyone else involved in growing, moving, managing and importing oak trees and plants must comply with these regulations. These are summarised in our good practice guide for handling oak material in areas affected by OPM, available from the Oak Tree Owners’ OPM Manual.
Legislation governing OPM in the UK is published on the government’s legislation website.
The legal requirements for importing oak trees and plants to the UK from the European Union are set out on GOV.UK.
Origin and history
OPM is a native of central and southern Europe, where predators and environmental and ecological factors usually keep its numbers in check and minimise its impact. However, its range has been expanding northwards over the past 20 years. The expansion has been aided by the movement of live oak trees in trade which might have OPM present on them, and perhaps also by a warming climate. It is now established as far north as The Netherlands and northern Germany, and has occasionally been seen in Sweden.
OPM was first accidentally discovered in Britain, in West London, in 2006. The introduction probably occurred during the autumn of 2005, the winter of 2005-06, or the spring of 2006, and almost certainly as eggs which had been laid on live oak plants before they were imported from continental Europe. The adults which emerged in 2006 laid eggs in nearby oak trees, establishing a local breeding population.
The current distribution of the pest has arisen from that and a small number of subsequent, similar introductions, and by natural spread from the original points of introduction. If it continues to spread, it might eventually colonise many other parts of England and Wales.
Small, separate introductions of OPM also occurred in Leeds in 2009 and Sheffield in 2010, and live caterpillars were intercepted on a freshly planted oak tree in Wiltshire in 2018. Eradication action was taken at all three sites, and follow-up surveys found no evidence that these introductions resulted in the establishment of breeding populations. We continue to monitor the Wiltshire site and surrounding area.
To contact the Forestry Commission’s OPM team:
- Email: firstname.lastname@example.org
- Tel: 0300 067 4442
To report a sighting of OPM: