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Good practice guide for handling oak material in areas affected by oak processionary moth (OPM)

This document provides guidance on good practice when working on trees in or close to areas affected by oak processionary moth (OPM), and when dealing with the tree material arising from work on OPM-infested trees. Any person responsible for tree-work sites within the affected areas must follow this guidance when working on oak trees.


OPM (Thaumetopoea processionea) is native to central and southern Europe. It was accidentally introduced into the United Kingdom in 2005 on trees imported from continental Europe for a landscaping scheme in West London. There have been other accidental introductions, and spread from some points of introduction, since then.

Its larvae, or caterpillars, live mostly on pedunculate or English oak (Quercus robur), sessile oak (Q. petraea) and Turkey oak (Q. cerris) trees, feeding on the leaves. However, it can also be found on other oak species and, in rare cases, other species of broadleaved trees, but usually only when they run short of oak leaves to feed on. They only breed on oak trees.

It might not be possible to eradicate OPM in the United Kingdom, particularly in the Established area. However, the Forestry Commission and its partners are working hard to minimise its population, spread and impacts as much as possible.

We are concerned that OPM could become a serious forestry pest in this country if it is not controlled. That is because the caterpillars’ feeding can threaten the health of oak trees. Large populations can strip whole trees bare of leaves, leaving them vulnerable to other pests, diseases and stress events such as drought and flood.

In addition, oak woodlands could become less attractive as places to visit because of the human and animal health impacts. That is because OPM caterpillars have thousands of tiny hairs which contain an irritating protein called thaumetopoein, giving rise to its scientific name. Contact with the hairs can cause itching skin rashes, eye and throat irritations and, less commonly, breathing difficulties in people and animals. Contact can occur by touching the caterpillars or their nests, caterpillars can eject hairs as a defence mechanism, and they can be blown about by the wind.

Clearly, tree surgeons, forestry workers and others who work on or close to oak trees in the affected areas are among those most exposed to these risks.

They are also well placed to help us prevent or limit its spread, and this guide describes how they can do so.


‘OPM-affected areas’ means all trees and woodland lying within the areas of the country known to have OPM populations. These areas, mostly in and around London, are indicated on the distribution map. The map is kept up to date from the results of surveys which the Forestry Commission and its partners carry out every year to establish the whereabouts of OPM, supplemented from reports of sightings received from members of the public.

Arboricultural contractors and tree surgeons operating in OPM-affected areas should have a specific risk and / or COSHH assessment for OPM, with associated protocols, depending on whether they do or do not engage in OPM removal or work on trees with OPM. This must be communicated to all staff who work on affected trees.

Any person involved with pruning or removing oak trees in affected areas must therefore be familiar with the insect at all stages of its lifecycle, as well as ‘live’ and old, or ‘spent’, OPM nests. The OPM Manual on our website sets out the life-cycle, and includes photographs of OPM nests in some of their many shapes, sizes and situations.

It is extremely important that before starting work on any oak tree in the OPM-affected areas you visually assess it to determine whether OPM is present. Eggs spend the winter on the trees, and the caterpillars emerge the following spring, usually about mid- to late April, and sometimes earlier.

There is a greater risk of spreading OPM when the caterpillars are emerging and are still small and difficult to spot, so much greater vigilance is required in the months of March, April and May. As they grow, the caterpillars feed, build nests and descend lower in the oak trees, and this is when they are most likely to be seen.

You might come across old, ‘spent’ OPM nests when working on oak trees outside the OPM season (spring and summer). Even though these nests are spent, the tree is still considered infested. That is because OPM females which emerged from the nests are highly likely to have laid their eggs on branches high in the tree’s canopy, so movement of material from that tree needs to be treated with caution.

Health and safety

We strongly recommend that people working on or close to OPM-infested trees take steps to protect themselves from exposure to OPM hairs. These include wearing the appropriate personal protective equipment (PPE), and other measures. Repeated exposure can result in the symptoms getting worse each time. Occupational health guidance is provided in the OPM Manual at

Reporting sightings

  • You are legally required to inform us immediately if you see evidence of OPM in any tree
  • You must notify us immediately even if you have already started work on the tree before seeing the OPM evidence
  • You must stop work on the tree as soon as it has been made safe. The area should be cordoned off and signed to warn others that OPM is present. Work should not resume until eggs, nests and caterpillars have been appropriately removed and destroyed
  • Making a dangerous tree safe takes priority, and in such cases you may continue work until the tree is safe. However you must report it as soon as possible.

Please report suspected OPM infestations by:

Note that TreeAlert requires photographs to be uploaded, and we prefer emailed reports to also include photographs. They should be close-up, in focus and well-lit, but do not risk contact with the hairs to get them.

If reporting sightings by email or telephone, please include one or all of the following:

  • the What3Word location of infested tree(s)
  • a precise location of the tree(s) – a 10-digit Ordnance Survey grid reference is ideal (e.g. AB12345 12345). Otherwise provide a full address, including property name and/or street or road number and the full postcode
  • precise instructions for finding the tree(s), e.g. “40 metres north-west of the entrance to [name] Park in [name] Street”.

Please also include:

  • a telephone number/email where we can reach you during the daytime to clarify any points
  • a clear, well-lit photograph with email reports if you can
  • contact details of the owner or manager of the tree(s), if known.


Anyone working on OPM-infested trees must follow the standard operating procedure (SOP) of the London Tree Officers’ Association (LTOA).

See also ‘Regulations’ below for information about the regulations which apply to work on OPM-infested trees.

NOTE: Do not presume that, because you cannot see any evidence of OPM, the tree is free of it – adopt the precautionary principle and use PPE. Minimum PPE specifications for OPM protection are set out in the SOP above and in our OPM Manual.


There are some regulations applying to the handling of OPM-affected material.

  1. Oak material smaller than 10 cm 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.
  2. Material greater than 10 cm in diameter, which is more likely to have OPM nests attached, poses less risk of spreading OPM. (10 cm 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.
  3. If oak material must be moved outside the OPM-affected areas, you must first consult the Forestry Commission OPM team at the contact details below.
  4. If arisings from a tree are to be moved outside the known OPM infestation area, all material less than 10 cm in diameter must be thoroughly chipped on site before being moved to an approved incinerator.
  5. 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.
  6. Storage of oak material before transport to an incineration or processing plant must last as short a period as possible.


Contact details

If you need to move infested oak material out of the OPM-affected areas, you must contact the Forestry Commission OPM team on the details below for advice about its safe and compliant removal.

Similarly, contact the OPM team if you have any questions which are not answered.

Oak Processionary Moth team

Forestry Commission

Bucks Horn Oak



GU10 4LS


For further information about OPM, including aids to identification and our manual for oak tree owners in affected areas, visit


Biosecurity: Keep it Clean

Prevent the introduction and spread of tree pests and diseases!

We can significantly reduce the risk of introducing and spreading tree pests and diseases by implementing appropriate biosecurity measures. For further information visit the Forestry Commission’s biosecurity page on

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