Managing oak processionary moth in England
Information on oak processionary moth (OPM) for those who own or manage oak trees, including private tree and woodland owners as well as local authorities.
Applies to England
Oak processionary moth (OPM) (Thaumetopoea processionea) is regulated as a quarantine pest known to be present in Great Britain. As a quarantine pest, it is under official control measures and cannot be introduced, moved, multiplied or released without prior authorisation.
If you own or manage oak trees, use this guide to identify and survey the OPM caterpillars and nests. If you find a nest or caterpillars, you must report it immediately via TreeAlert.
This guide details information on government management of OPM, and restrictions on the trade and movement of oak trees.
If you have oak trees on your land, or you work on oak trees, read how to plan and manage risks from OPM. You should also refer to:
- Moving large oak trees within the Oak Processionary Moth (OPM) management zones.
- Updated management Zones for Oak Processionary Moth, March 2024.
Oak processionary moth (OPM) caterpillars processioning up an oak tree trunk. Image credit: Henry Kuppen.
What is OPM?
The caterpillars of OPM infest oak trees. They eat the leaves, which weakens the tree and leaves it vulnerable to other threats.
The nests and caterpillars are also a hazard to human and animal health.
OPM was first identified in London in 2006 and spread to some surrounding counties in south east England. See the latest distribution map and check the OPM to see which management zone your trees are in.
The moths spread by flying from one tree to the next. Usually staying close to the tree they originated from.
How to identify OPM
Caterpillars
The caterpillars have black heads and grey bodies covered in long white hairs. See these example .
An army of oak processionary moth (OPM) caterpillars huddled on an oak leaf. Crown copyright.
The caterpillars are 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, like their name suggests. You may be able to see them on all parts of the tree, on the trunk, branches and leaves, and occasionally on the ground.
Nests
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.
A white and fluffy OPM nest on an oak tree branch. Crown copyright.
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. Check the tree 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 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 summer and into autumn.
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
You can find used nests at any time of year, but they are easiest to spot in winter when there are no leaves on the oak tree or on the ground.
Species commonly confused with OPM
We often receive reports of caterpillars that are not OPM.
Check the guide for distinguishing between OPM caterpillars and other species before reporting a sighting.
How to report sightings of OPM
Sightings should be reported using Tree Alert.
You can also email opm@forestrycommission.gov.uk, but Tree Alerts are preferred.
Find out what to include in your OPM report on the OPM resource hub.
We may issue a Statutory Plant Health Notice (SPHN) asking you to take action.
We’ll take charge of treating infested trees in the buffer zone and the area free from the pest. You can view these areas on the .
Removing nests and caterpillars is a hazardous operation. We recommend that a professional does this work. Find an OPM management professional at the Arboricultural Association.
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 when they brush past an infested oak tree or if the hairs are blown onto them by the wind from nearby infested trees.
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. 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. The risk is possible at any time of year due to residual hairs from old nests.
Read the for more safety information and details on identification and control of OPM.
Reduce risk to people and animals
Make sure you, and people or animals in your care, do not touch or go near nests or caterpillars.
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 professional 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.
Further advice is available from the NHS website and Blue Cross for pets.
Management in the OPM zones/areas
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 3 areas or zones described below.
Area free from the pest
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 area free from the pest, the government will take robust actions to eradicate it.
Buffer zone
This area has an annual OPM programme of surveillance and control led by the 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, you may be issued with a Statutory Plant Health Notice (SPHN) asking you to take action. In most cases support will be available from the Forestry Commission for treatment of trees within this zone. The SPHN will explain what action is needed.
Established area
This is the area where OPM is established in the south east of England. Landowners are responsible for the management of OPM on their land, however guidance is available on the OPM hub.
If you own infested oak in this area it is recommended you find a professional on the Arboricultural Association website to treat and remove any hazardous nest material if necessary. You should still check for OPM every year.
OPM management zones
In 2023 management zone boundaries were introduced. To help landowners plan OPM management for 2024, read the updated .
See the that fall into the buffer zone and established area. It is important to check this list as some wards may be included in the management zones, but not the entire local authority area.
You can also use the Forestry Commission interactive map to see what zone you’re in, or download the full (buffer zone, established area and 10km exclusion zone).
Restrictions on trade and movement of oak trees
Imports 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 legislation requires that imports into Great Britain from EU countries can only take place if the oak trees have been grown throughout their life:
- in places of production in countries in which OPM is not known to occur
- in a Pest Free Area for OPM, established by the national plant protection organisation in accordance with ISPM number 4
- in a site with complete physical protection against the introduction of OPM and has been inspected at appropriate times and found to be OPM free
The legislation applies to oak trees (Quercus L), with a girth (circumference) 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.
Movement of oak trees in Great Britain
Restrictions on moving large oak trees (Quercus L), with a girth (circumference) at 1.2m above the root collar of 8cm (2.55cm diameter approx.) in Great Britain vary dependent on what OPM management zone the trees are in.
The Great Britain OPM was updated for the start of the OPM 2024 season. You can also see a that fall into the buffer zone and established area.
To see what zone the large oak trees you want to move are in, and what zone they would be moved to, you can use the Forestry Commission interactive map or download the full (buffer zone, established area and 10km exclusion zone).
Legislation came into force on 24 May 2023 that establishes a demarcated area and enables a risk-based approach to moving large oak trees within the buffer zone and established area.
This is summarised in the guide: Moving large oak trees within the oak processionary moth (OPM) management zones.
You can also watch our video which explains the policy changes.
Professional operators growing or moving large oak trees who are impacted by the movement restrictions should consult with their local plant health inspector or the Plant Health and Seeds Inspectorate for further advice.
Moving large oak trees from the area free from the pest
Large oak trees in the area free from the pest can move anywhere in Great Britain.
Moving large oak trees into the area free from the pest
Movement of large oak trees from the buffer zone or established area into the area free from the pest is prohibited unless they 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.
Moving large oak trees within the buffer zone and established area
No movement of large oak trees are permitted from the established area into the buffer zone.
No movement of large oak trees are allowed from the established area or buffer zone into a 10km exclusion zone at the outer edge of the buffer zone boundary to protect the area free from the pest see: or download the full (buffer zone, established area and 10km exclusion zone).
Large oak trees can be moved by professional operators within the established area, or into the established area from the buffer zone, provided the following 2 biosecurity requirements are met:
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Biosecurity competence: professional operators must demonstrate that they meet the Plant Health Management Standard, evidenced through membership of the Plant Healthy Certification Scheme or provision of a Ready to Plant approval from Fera Science Ltd for each consignment to be moved, as well as evidence of ongoing on-site monitoring for OPM and inspection of trees for OPM prior to movement.
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Official inspections: professional operators must keep accurate records of the contact details of those receiving large oak trees, including the delivery address and contact details, and store this information for a minimum of 3 years to ensure traceability of movements. This information can be recorded using the .
Large oak trees can be moved within the buffer zone if they meet an additional 2 biosecurity requirements. These are:
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Applying phytosanitary treatments: professional operators must have a robust control regime in place with appropriate application of phytosanitary treatments in line with Defra’s technical guidance on the application of Plant Protection Products for phytosanitary treatment of OPM. Registration is required for professional users of plant protection products under the Official Control (plant protection products) Regulations. Records of all treatment applications must be kept for a minimum of 3 years.
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Post planting inspections: a proportion of high-risk large oak movements within the buffer zone will be subject to post-planting inspections by the plant health inspectors. Therefore, professional operators must disclose the potential for post-planting inspection as part of the new legislation to buyers at the time of purchase.
The biosecurity requirements do not apply if the large oak trees are in transit for less than 48 hours before moving to the final planting site. You must still record the final destination of the large oak trees for tracing purposes.
The biosecurity requirements also do not apply if the large oak trees you want to move have been grown throughout their entire life with complete physical protection against the introduction of OPM and have been inspected at appropriate times and found to be free.
Handling oak material in OPM affected areas
‘Oak material’ is material arising from tree works, or cut branches. This excludes movement of live oak trees.
OPM-affected oak material should be carefully handled to avoid further distribution of the pest:
- it is important that before starting work on any oak tree in the OPM affected areas you visually assess it to determine whether OPM is present
- smaller oak material, such as branch and brash wood, is most likely to harbour OPM eggs, this must not be moved outside the area of OPM infestation unless absolutely necessary. It should be retained on site if possible for example as wood chip
- larger oak material, such as logs are more likely to have OPM nests attached, they pose less risk of spreading OPM but could be a risk to those working on the oak tree
- if arisings from a tree are to be moved outside the known area of OPM infestation, all small material 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 affected oak material before transport to an incineration or processing plant must last as short a period as possible
Funding and support for oak with OPM
In 2024 there are 2 pilot schemes within the established area:
Private residents (applications are closed)
Support to cover the cost of treatment of a small number of oak trees. This offer is for people living in residential properties such as houses or flats (private residents) who have 4 or fewer oak trees, which are at least 2m tall, in their garden. The grant will help fund the spraying of trees in spring as part of the government’s wider control programme. Forestry Commission control contractors arrange and carry out the spraying.
Local authorities
A local authority grant for oak with OPM is available through the Tree Health Pilot scheme. It supports local authorities in the OPM ’established area’ to organise surveying of oak trees for OPM and create an OPM management plan and communications strategy to understand the risks and appropriate management of the affected area.
The grant also offers funding for communications materials such as leaflets and signage and biosecurity items including boot cleaning equipment and red tape/posts for cordoning off areas.
Read the Tree Health Pilot case study and learn how Haringey Council used funding from the scheme for proactive management of the pest.
Find out if you’re located in the OPM established area on the latest OPM distribution map.
Apply for a grant as a group
If you, someone you know, or an organisation are interested in applying to lead a group application, email thpilotenquiries@forestrycommission.gov.uk.
Specialist advice is available to help you with your application and to advise on managing oak trees with OPM, the risks they pose, and any related questions you may have. Experience of working with and knowledge of the pest may be helpful, but is not necessary.
Further information about OPM
Read OPM: plan and manage risks.
Read the Tree Council guidance for local authorities and larger landowners on preparing an action plan to manage OPM.
Protected sites
If your site is subject to a designation such as Site of Special Scientific Interest (SSSI), or might contain European Protected Species, additional rules apply.
Read guidance from Natural England.
Latest resources:
- Tree Health Pilot case study
- latest OPM distribution map
- OPM management policy changes guide
- Get the Forestry Commission’s OPM programme newsletters
- the OPM information hub has tools to help manage OPM including how to develop an OPM management plan, general information and advice, and an interactive map showing where OPM is present
Updates to this page
Published 14 January 2022Last updated 20 November 2024 + show all updates
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Added a link to the Tree Health Pilot case study.
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Updated the section on what help is available.
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Added a full list of postcodes in the OPM management zones. Addition of sub heading on moving oak trees.
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Added a new management zones map 2024 with exclusion zone, and a section on latest resources.
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Information about OPM regulations and reporting OPM added as a call to action.
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Applications for OPM treatment offer are now closed.
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Updated the section 'What support is available' with an opening for support for private residents with oak trees.
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Added new link to the Defra guidance on the application of Plant Protection Products for phytosanitary treatment of oak processionary moth, which was updated to reflect changes to authorised use of DiPel DF (Bacillus thuringiensis).
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Update to: Guidance on application of phytosanitary treatments for the control of Oak Processionary Moth (Thaumetopoea processionea) 2023.
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Updated at the new management regulations have come into force.
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Updated to include new map and guidance.
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Updating the page to include the 2023 enforcement area map.
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Updated information regarding OPM.
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Added: guidance for distinguishing between OPM caterpillars and those of other species.
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Addition of public information leaflet.
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General page updates
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Updates on how to identify OPM
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First published.