Managing oak processionary moth in England

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.

You will also find information on government management of OPM and restrictions on the trade and movement of oak trees.

What is 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 2023 with exclusion zone (PDF, 445 KB, 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

Caterpillars

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 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.

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 summer 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

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) for distinguishing 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. We also accept reports via email at opm@forestrycommission.gov.uk, although TreeAlerts are preferred. Please read the guidance regarding the essential information to be included in OPM reports on the OPM resource hub.

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 2023 with exclusion zone (PDF, 445 KB, 1 page). These areas have been updated and came into force on 8 March 2023.

Removing nests and caterpillars is a hazardous operation. We therefore 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 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 residual 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.

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.

Further advice is available from the NHS website and Blue Cross for pets.

Managing 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.

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, government will take robust actions to eradicate it.

Buffer zone

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. In most cases support will be available from the Forestry Commission for treatment of trees within this zone. The SPHN document will explain what action is needed.

Established area

This is the area where OPM is established in the South East of England. 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 necessary. You should still check for OPM every year.

OPM Management Zones

In response to findings of OPM in 2021/2022, updated management zone boundaries were introduced on 8 March 2023. To help landowners plan OPM management for 2024, please refer to the updated management zones map 2023 with exclusion zone (PDF, 445 KB, 1 page).

A list of local authorities and wards (ODT, 62.6 KB) that fall into the buffer zone and established area is also available. It is important to check this list as this year some wards may be included in the management zones, but not the entire local authority area.

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 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 also requires that imports into Great Britain from EU countries 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 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.

Large oak trees in the area free from the pest can move anywhere in Great Britain.

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.

The current Great Britain OPM management zones map 2023 with exclusion zone (PDF, 445 KB, 1 page) is available to view and was updated for the start of the OPM 2023 season. A list of local authorities and wards (ODT, 62.6 KB) that will fall into the buffer zone and established area is also available.

Movement of oak trees within the Buffer zone and Established area

May 2023

New legislation came into force on 24 May 2023 that establishes a demarcated area around the buffer zone and established area. This enables a risk-based approach to moving large oak trees (girth at 1.2m above the root collar of 8cm or more) 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: management zones map 2023 with exclusion zone (PDF, 445 KB, 1 page).

Large oak trees are able to be moved by professional operators within the established area, or into the established area from the buffer zone, provided the following two biosecurity requirements are met:

1. 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.

2. 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 three years to ensure traceability of movements. This information can be recorded using the Post planting Inspections form (MS Excel Spreadsheet, 50.2 KB).

Large oak trees are able to be moved within the buffer zone if they meet an additional two biosecurity requirements. These are:

1. 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 oak processionary moth. 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.

2. 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.

To see what zone the large oak trees you want to move are currently in, and what zone they would be moved to, you can use the Forestry Commission interactive map and search by postcode. Further updates were made to the interactive map to include the exclusion zone in May 2023.

Any 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.

Handling oak material in OPM affected areas

Guidance on movements of OPM affected material i.e. cut branches or arisings from tree works (not movement of live oak trees).

OPM-affected oak material should be carefully handled so as not to result in 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

What support is available

In 2024 there are 2 pilots within the established area:

1. Support for private residents to cover the cost of treatment of a small number of oak trees. This offer is for residents (living in residential properties such as houses or flats) who have 4 or fewer oak trees, which are at least 2m tall, in their garden. The grant will help fund the spraying of residents’ trees in spring 2024 as part of the government’s wider control programme. Forestry Commission control contractors will arrange and carry out the spraying. Applications for this offer have now closed.
2. OPM Groups Grant: this grant, available through the Tree Health Pilot scheme, supports a facilitator to form a group of land managers in the OPM Established Area, organise initial surveying of oak trees for OPM and create an OPM management plan to understand the risks and appropriate management of the affected area. The grant also offers funding for signage and biosecurity items including boot cleaning equipment and red tape/posts for cordoning off areas, as well as an hourly fee for time spent acting as a facilitator for the group.

If you, someone you know, or an organisation might be interested in applying to lead a group application, please contact 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

• 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.