Oak processionary moth: what to look for and what to do

The caterpillars' hairs cause rashes, sore eyes and breathing problems. Here's how to spot a nest and why you shouldn't touch it.

Oak processionary moth (OPM) is a pest of oak trees that is now established across London and spreading into surrounding counties, including Essex and Hertfordshire. The caterpillars strip oak leaves, but the bigger problem is people's health.

Why it matters

Older caterpillars carry thousands of tiny hairs containing a protein that irritates skin, eyes and airways. The hairs blow on the wind and stay active in old nests for years. Contact can cause itchy rashes, eye irritation and, less often, breathing difficulties.

How to spot it

• Nests: white, silky webbing on oak trunks and branches, often the size of a fist or larger, turning brown with age.
• Caterpillars: grey bodies with long white hairs and a dark stripe, moving in nose-to-tail lines, which is how they got the name.
• When: caterpillars are active from roughly April to July. Nests stay hazardous long after.

What to do

1. Keep people, children and pets away. Don't touch, brush or try to burn the nest.
2. Report it to the Forestry Commission through the TreeAlert online service.
3. Get a qualified contractor to remove it. Removal needs protective clothing and proper disposal.

How we help

We inspect oak trees as part of our pest and disease surveys and our commercial site inspections. If we find OPM we'll explain the options for removal and long term control, and handle the reporting.