What is Oak Processionary Moth?

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.