Where in the UK is Oak Processionary Moth found?

OPM is established across much of Greater London and parts of the surrounding counties, with a wider buffer zone subject to monitoring and occasional outlying findings elsewhere. Distribution changes over time, so current official guidance should always be checked.

How did OPM arrive in the UK?

It was accidentally introduced, associated with imported live oak trees, and was first noted as established in London in the mid-2000s. Movement of oak planting stock remains a recognised pathway, which is why plant health controls exist.

Does OPM kill oak trees?

Rarely on its own. Heavy repeated defoliation weakens trees and leaves them more vulnerable to drought and other pests and diseases, but a healthy mature oak generally recovers from a season's feeding.

Is OPM a notifiable pest?

OPM is subject to official controls and monitoring in the UK, and suspected sightings should be reported through TreeAlert. Requirements can change, so check current Forestry Commission and GOV.UK guidance for your area.

Understanding OPM

What is Oak Processionary Moth?

A non-native moth, established in parts of the UK, whose caterpillars are simultaneously a tree health issue and a public health concern.

The species

Oak Processionary Moth (Thaumetopoea processionea) is a moth native to central and southern Europe. In the UK it was accidentally introduced on imported oak planting stock and became established in London in the mid-2000s. It is now present across much of Greater London and parts of the neighbouring counties, with monitoring extending well beyond that area.

Why it matters

The caterpillar stages carry very large numbers of microscopic barbed hairs containing an irritant protein. These hairs are shed, become airborne in dry or windy conditions, and accumulate in nests and shed skins. Contact can cause itching skin rashes, eye irritation, sore throats and, less commonly, more significant allergic or respiratory reactions.

That combination — a tree pest that is also a human and animal health hazard — is what makes OPM a site management issue rather than purely an arboricultural one. The same infestation can be almost irrelevant in remote woodland and highly significant beside a school playground.

Impact on oak trees

Feeding produces skeletonised leaves and, in heavier years, visible defoliation of parts of the canopy. Mature oaks in reasonable condition usually recover from a single season. The concern is cumulative: repeated defoliation, especially alongside drought stress or other pests and diseases, adds pressure to trees that may be centuries old.

Where it is found

OPM is associated with oak species. Distribution is monitored nationally, with control and movement requirements applying in defined areas. Because those boundaries are reviewed periodically, the current position should always be confirmed against Forestry Commission and GOV.UK guidance rather than assumed.

Oak foliage showing skeletonised leaves from caterpillar feeding — Feeding damage: skeletonised leaves and patchy canopy thinning. Illustrative image — not a scientific reference photograph.

At a glance

Host tree: Oak species
Risk stages: Caterpillar stages and nests, including old nests
Peak visibility: Approximately May to July
Adult moth: Harmless, nocturnal, rarely seen

How to identify OPM Health risks explained Management approaches

Life cycle

One generation a year, tracked to the oak

Each stage has different implications for what you can see and what should be done.

Late summer to early spring

Egg stage

Eggs sit in camouflaged plaques on twigs high in the canopy through autumn and winter. They are extremely difficult to see from the ground.

What this means for inspection

Winter is when climbing inspection or close canopy assessment can locate egg plaques, and when old nests are easiest to spot on bare branches.

Official reporting

Found Suspected OPM?

Suspected Oak Processionary Moth sightings should be reported through the official TreeAlert tree pest and disease reporting service.

Our identification service provides preliminary guidance and does not replace official reporting.

Report OPM to TreeAlert Need help identifying it?

Background questions

Supported by

OPM Advice & Management UK is supported by Hertfordshire Pest Control, who provide the professional inspection, monitoring and nest management capability behind the advice on this site.

OPM Advice & Management UK is an independent information resource and is not part of, affiliated with or endorsed by Defra, the Forestry Commission, Forest Research or any local authority. Official organisations and resources are referenced to help visitors access authoritative information. Suspected OPM sightings should be reported through the appropriate official reporting channels.