The oak processionary moth (OPM) is a tree pest and a health hazard affecting oak trees across parts of the UK. First introduced accidentally in 2005, it is now established mainly in Greater London and surrounding counties, with government programmes in place to limit its spread.

OPM caterpillars feed on oak leaves, weakening trees and making them more vulnerable to other diseases and environmental stress. The caterpillars also carry tiny toxic hairs that can cause skin rashes, eye irritation, sore throats and breathing difficulties in people and animals. The risk is highest in May and June, when the caterpillars are most active.

Residents are urged not to touch the caterpillars or their white silken nests, and to keep children and pets away from affected trees. Sightings should be reported via TreeAlert, with a clear photograph if safe to obtain.

The Forestry Commission provides detailed guidance on identification, health precautions and reporting. You can read the full information here.