A garden pest with legal status
Oak processionary moth (OPM) is unlike most garden caterpillars. GOV.UK guidance from the Forestry Commission describes it as a regulated quarantine pest known to be present in Great Britain, which cannot be introduced, moved, multiplied or released without authorisation. The guidance summarised here, “Managing oak processionary moth in England”, was updated in August 2026 and is written for England. Scotland, Wales and Northern Ireland have their own plant health guidance, which is outside the scope of this article. It is general information, not health or legal advice.
What it is
The caterpillars of OPM infest oak trees and eat the leaves, which weakens the tree and leaves it vulnerable to other threats. The government says OPM was first identified in London in 2006 and spread to some surrounding counties in south-east England. The moths spread by flying from tree to tree, usually staying close to the tree they came from.
How to identify the caterpillars and nests
GOV.UK describes the caterpillars as having black heads and grey bodies covered in long white hairs. They are about 2mm long when they emerge in spring, tend to stay high in the trees until older, develop the irritating hairs when they reach about 1cm, and are fully grown at about 2cm. They usually move nose to tail in a procession, which gives the species its name.
Nests are usually found in early summer on the trunks and branches of oak trees, at any height. They are typically dome or teardrop shaped, ranging from the size of a golf ball to a rugby ball. Fresh nests are white with silken trails, but they soon discolour to brown and can stay on the tree for many months after the adult moths have emerged. GOV.UK says nests are unlikely to be found on other tree species, fences or walls. The guidance also warns that many reports involve caterpillars that are not OPM and refers to a guide to species commonly confused with it.
When and why they matter
Caterpillars emerge from late March to May. From late June to early August they retreat into the nests and form pupae, which stay there until the adult moths emerge. Late spring and early summer are the best times to look for caterpillars and new nests, and used nests are easiest to spot in winter.
The health risk is the reason the guidance is so firm. GOV.UK says the caterpillars and nests contain hairs that can cause itchy rashes, eye and throat irritation and occasionally breathing difficulties in people and animals, and that in rare cases they may cause an allergic reaction. It adds that repeated exposure can sensitise a person, worsening symptoms. Hairs can reach people who brush past an infested oak tree or be blown onto them from nearby trees, and animals can pick up hairs on paws or by sniffing and licking. The greatest risk is from May to July, though residual hairs from old nests mean that risk is possible at any time of year.
What to do if you suspect OPM
The advice is not to touch or go near the caterpillars or nests. Anyone who finds a nest or caterpillars must report it immediately through the online tool TreeAlert, which is the preferred route, or by email. The Forestry Commission may issue a Statutory Plant Health Notice asking the owner to take action.
GOV.UK says removing nests and caterpillars is a hazardous operation and recommends that a professional does it, suggesting the Arboricultural Association as a place to find an OPM management professional. Anyone who works on or close to oak trees in affected areas needs to use protective measures such as personal protective equipment.
For people or animals who have been exposed, the guidance says to get medical help for a serious allergic reaction, that a pharmacist can advise on relief from skin or eye irritation, and that a vet should be consulted if an animal is seriously affected. It advises telling the medical professional or vet that OPM contact is suspected.
The management zones
Government management depends on where a tree is. Most of the country is an area officially designated free from the pest, where government-funded surveillance operates and any outbreak is met with action to eradicate it. A buffer zone has an annual programme of surveillance and control led by the Forestry Commission, and an owner of an infested oak there may be issued with a Statutory Plant Health Notice. An exclusion zone, a 10km-wide strip on the outer ring of the buffer zone, has additional movement restrictions. In the “established area” in south-east England, landowners are responsible for managing OPM on their land, and the guidance says an owner of infested oak should find a professional on the Arboricultural Association website.
The zone boundaries and postcode lists are published by the Forestry Commission, and households can check which zone applies to them before deciding what to do.
What this means for pest control quotes
Because the moth is a regulated pest with a reporting duty, the correct first step for a suspected OPM problem in England is to report it and check the zone, not to book the cheapest treatment. A contractor who offers to remove nests without mentioning protective equipment, reporting or the zone rules is not following the approach in the guidance.
The bottom line
Oak processionary moth is a regulated quarantine pest whose hairs can cause skin, eye and breathing problems. In England, a suspected nest or caterpillar should be reported immediately, should not be touched, and should be dealt with by a professional where removal is needed. The management zone determines who is responsible for treatment.
Sources
- Forestry Commission, GOV.UK, “Managing oak processionary moth in England”
- Forestry Commission, GOV.UK, “Oak processionary moth: Plan and manage risks”