Oak processionary moth (OPM) (Thaumetopoea processionea)
If you own or manage oak trees, you should use this guidance to identify and survey the caterpillars and nests, and report any sightings.
What is the OPM?
The caterpillars of OPM infest oak trees, eating (defoliating) the leaves, weakening the tree and leaving it vulnerable to other threats. OPM nests and caterpillars are also a hazard to human and animal health.
OPM was first identified in London in 2006 and has spread to some surrounding counties including Surrey and other parts of south east England. See the latest distribution map and check the OPM management zones map (PDF, 1.36 MB, 1 page) to see which management zone your trees are in.
OPM moths spread by flying from one tree to the next. Normally they stay close to the tree they were on as a caterpillar.
How to identify OPM
The caterpillars have black heads and grey bodies covered in long white hairs. Please see these example OPM caterpillar images (PDF, 358 KB, 1 page).
The caterpillars are only about 2mm long when they emerge in Spring, and tend to remain high in the trees until they are older and larger. When they reach 1cm long they develop the irritating hairs. They’re fully grown at 2cm long.
They usually move nose-to-tail in a procession, hence their name. You may be able to see them on all parts of the tree - on the trunk, branches and leaves, and occaisonally on the ground.
Nests are usually found in early summer, in the trunks and branches of oak trees. A tree or branch can contain many nests. They can appear at any height - from the ground to the top of the tree.
You are unlikely to find nests on any other tree or shrub species, fences, walls or other similar structures (apart from structures supporting the tree).
They are typically dome or teardrop-shaped, and range from the size of a golf ball to a rugby ball. When fresh, the nests are white with white silken trails made by the caterpillars along the branches and leaves. But the nests soon become discoloured and brown, making them harder to spot.
Nests stay attached to the tree for many months after the adult moths have emerged.
When to look for OPM
Late spring and early summer are the best times to spot OPM caterpillars and their new nests in your oak trees. You can look for evidence of OPM from the ground, best checking from several angles. Using binoculars for this is recommended.
Spring and summer
The OPM caterpillars emerge from late March to May. They’ll move down the trees as they get older and bigger, and build nests.
Look for new nests in late spring and early summer when the caterpillars are active. Focus on the branches and trunks at all heights, and the ground for fallen nests later in the season.
From late June to early August, the caterpillars retreat into the nests and form a pupa (cocoon). The pupae remain in the nests until they emerge as adult moths.
Autumn and winter
Look for used nests in winter as they are easier to spot when there are no leaves on the oak tree or on the ground, but you can find them at any time of year.
Species commonly confused with OPM
We often receive reports of caterpillars which are not OPM, please take time to check this guidance Species commonly confused with oak processionary moth (OPM) (PDF, 5.17 MB, 1 page) fordistinguishing between OPM caterpillars and those of other species before submitting any reported sighting.
How to report sightings of OPM and what happens after you file a report
Any sightings should be reported to the Forestry Commission via the Tree Alert online form. Alternatively, people can email firstname.lastname@example.org or call 0300 067 4442.
The Forestry Commission may issue you with a Statutory Plant Health Notice (SPHN) asking you to take action. This document will explain what action is needed.
In general, the Forestry Commission will take charge of treating infested trees in the buffer zone and pest free area. You can view these areas on the management zones map (PDF, 1.36 MB, 1 page). These areas come into force on 1 March 2022.
Removing nests and caterpillars is a hazardous operation. We therefore recommend that a professional does this work. You can find a professional on the Arboricultural Association website. They may spray the trees with an approved bio-pesticide or insecticide.
Risks of OPM
Do not touch or get close to the caterpillars or nests.
OPM caterpillars and nests affect the health of people and animals, as they contain hairs which can cause itchy rashes, eye and throat irritations and occasionally breathing difficulties in both people and animals.
In rare cases the hairs may cause an allergic reaction. You can become sensitised by repeated exposure to the hairs, worsening your symptoms.
People or animals can come into contact with OPM hairs if they touch OPM caterpillars and their nests by brushing past an infested oak tree or are near infested trees as hairs can be blown about in the wind.
Animals can get hairs on their paws, and hairs can also get into their mouth and nose by sniffing, licking and picking up caterpillars or nests causing irritation. Livestock and horses can come into contact with hairs on the grass or other plants they eat, and by grazing close to a fallen nest.
The greatest risk is between May to July when the caterpillars are active. Although the risk is possible at any time of year due to hairs from old nests.
A public information leaflet covering the key facts, identification and control of OPM is available: Oak Processionary Moth - public information leaflet (PDF, 469 KB, 2 pages).
Reduce risk to people and animals
To avoid contact with the hairs, make sure you, and people or animals in your care, do not touch or go near nests or caterpillars.
Removing nests and caterpillars is a hazardous operation, it is recommended you call a suitably qualified arborist or pest control expert to remove infestations from your trees.
If you work on or close to oak trees in the affected areas, you need to take care and use protective measures, such as personal protective equipment (PPE). Read the guide on how to plan and manage risk of OPM for more information on controlling it and the PPE you need.
What to do if exposed
If you or someone in your care has a serious allergic reaction, get medical help. For less severe reactions, a pharmacist can provide advice on relief from skin or eye irritations.
If an animal in your care is seriously affected, get advice from a vet.
Tell the medical person or vet you suspect it is due to OPM contact.
You should report OPM via the Tree Alert online form, and let your neighbours who have oak trees know that they might also have an OPM infestation.
How to manage OPM
Forestry Commission leads an annual programme of work to manage OPM. The actions required under the programme will vary depending on where your site is located.
There are currently three areas or zones described below.
Pest free area
This area is officially designated free from the pest and covers most of the country. In this area government funds an extensive programme of surveillance to monitor for OPM. If OPM is found in the pest free area, government will take robust actions to eradicate it.
In this area there is an annual OPM programme of surveillance and control, led by Forestry Commission. This programme involves detecting the pest and treating infested oak trees (and those trees in close proximity). If you own an infested oak tree in the buffer zone, the Forestry Commission may issue you with a Statutory Plant Health Notice (SPHN) asking you to take action, and in most cases support is currently available for treatment of trees within this zone. The SPHN document will explain what action is needed.
This is the area where OPM is established. In this area landowners are responsible for the management of OPM on their land, however guidance is available on the OPM hub. If you own an infested oak tree in this area it is recommended you find a professional on the Arboricultural Association website to treat and remove any hazardous nest material if required. You should still check for OPM every year.
The OPM management zones are being updated for the start of the OPM 2022 season and a new map showing these zones can be found on the management zones map (PDF, 1.36 MB, 1 page) to help landowners plan OPM management. The management zones will come into force on 1 March 2022.
If you own or manage oak trees, you should use the Oak Processionary Moth: plan and manage risks guidance to help plan and manage the risks from OPM.
Restrictions on movements of oak material i.e. cut branches or arisings from tree works (not movement of oak trees).
There are some regulations applying to the handling of OPM-affected material.
Oak material smaller than 10cm in diameter, which is the material most likely to harbour OPM eggs, must not be moved outside the OPM-affected area unless absolutely necessary. It should be retained on site if possible until its movement no longer presents a risk of spreading the pest
Material greater than 10cm in diameter, which is more likely to have OPM nests attached, poses less risk of spreading OPM. (10cm is about the diameter of a baked bean can.) However, there is a general presumption and recommendation to manage larger material in the same way as for smaller material
If oak material must be moved outside the OPM-affected areas, you must first consult the Forestry Commission OPM team email@example.com or phone 0300 067 4442
If arisings from a tree are to be moved outside the known OPM infestation area, all material less than 10cm in diameter must be thoroughly chipped on site before being moved to an approved incinerator
Oak material being transported must be contained within an enclosed vehicle which prevents any material from escaping. The transporting vehicle must be washed down afterwards in a designated area with provision for preventing washings from entering any watercourse
Storage of oak material before transport to an incineration or processing plant must last as short a period as possible
Restrictions on trade and movement of oak trees
To protect the country against OPM through movement and imports, OPM is regulated as a quarantine pest and legislation is in place to protect oak trees against the pest.
No imports of oak trees from outside the EU are currently permitted.
The import of large oaks (which are susceptible to OPM) from EU countries is also prohibited, unless they originate in a pest free area, a country where OPM is not known to occur, or have been grown under physical protection throughout their life. The legislation applies to oak trees (Quercus L), with a girth at 1.2m above the root collar of 8cm (2.55cm diameter approx.) or more as these trees represent the greatest likelihood of introducing OPM.
The legislation requires that imports into the GB Pest Free Area from EU countries and movements within GB can only take place if the oak trees concerned:
- have been grown throughout their life in places of production in countries in which OPM is not known to occur
- have been grown throughout their life in a Pest Free Area for OPM, established by the national plant protection organisation in accordance with ISPM No. 4
- have been grown throughout their life in a site with complete physical protection against the introduction of OPM and have been inspected at appropriate times and found to be free
The requirements apply to movements within GB, and therefore movements of large oak trees with a girth at 1.2m above the root collar of 8cm (2.55cm diameter approx.) or more within the buffer and established areas and from the buffer and established into the Pest Free Area are prohibited unless they have been grown throughout their life in a site with complete physical protection and have been inspected.
From 2 March 2022 new legislation will come into force and the import requirements will be extended to apply to GB, rather than just the GB Pest Free Area.
The OPM management zones are being updated for the start of the OPM 2022 season. The new management zones map (PDF, 1.76 MB, 1 page) will come into force and replace the existing boundaries on 1 March 2022. The map includes a line showing the 2020 infestation outer extent. A list of local authorities (PDF, 43.6 KB, 1 page) in the buffer and established area is also available.
Any nurseries or traders growing oak trees with a girth at 1.2m above the root collar of 8cm (2.55cm diameter approx.) or more who are impacted by the new legislation (e.g. because they were previously in the Pest Free Area and now fall within the expanded buffer zone or because of the new movement restrictions), should immediately consult with the Plant Health and Seeds Inspectorate for advice
What support is available
In 2022 there are two pilots within the established area:
- support for private residents with small numbers of oak trees to help with costs of treatment of oak (now closed to new applications – we will be reviewing the support for private residents over the coming months and will update information later this year)
- OPM Groups Grant: This grant will support a facilitator to form a group in the OPM Established Area. The grant will support surveying of oak trees for OPM, understanding the risks and appropriate management of OPM. Workshops will be hosted by the Forestry Commission for groups to attend and learn from specialists about managing oak trees with OPM, the risks they pose as well as answering any questions they might have. If you, someone you know, or an organisation might be interested in applying to lead a group application, then please contact firstname.lastname@example.org.
Further information about OPM is available:
- Oak Processionary Moth information hub: the hub includes tools to help manage OPM including how to develop an OPM management plan, and sources of information advice, including an interactive map to help identify where OPM is in the buffer and established area.
- Oak processionary moth: Plan and manage risks
If a site is subject to a designation such as Site of Special Scientific Interest (SSSI), or might contain European Protected Species, additional rules apply. See guidance from Natural England.
Guidance is also available from the Tree Council for local authorities and larger landowners on preparing an action plan to manage OPM.
Advice about stings and bites is available on the NHS website.
You can also receive updates from the Forestry Commission’s OPM control programme, which are available throughout the season through regular OPM programme newsletters.
Published 14 January 2022
Last updated 14 July 2022 +show all updates
Added: guidance for distinguishing between OPM caterpillars and those of other species.
Addition of public information leaflet.
General page updates(Video) Tree health focus: Oak Processionary Moth
Updates on how to identify OPM