Oak processionary moth (OPM) – a growing risk to trees and public health
If you've walked past an oak tree in spring or early summer and noticed clusters of caterpillars moving nose-to-tail, you might have encountered one of the UK's most concerning tree pests - the oak processionary moth.
While small, this species poses a big threat - not just to oak trees, but also to people and animals. Here's everything you need to know.
What is the oak processionary moth?
This moth is an invasive species in the UK, originally from southern Europe. It was accidentally introduced into London in the mid-2000s and has since spread to parts of South East England.
The moth itself is relatively harmless, but its caterpillars are the real problem. The larvae live and feed almost exclusively on oak trees, move in distinctive head-to-tail “processions” and build white, silky nests on trunks and branches
Why are they a problem?
Damage to trees - oak processionary caterpillars feed on oak leaves, sometimes stripping trees bare. Repeated loss of foliage can weaken trees, reduce growth and make them more vulnerable to disease and environmental stress.
Risk to human and animal health
Along with the damage to trees, the caterpillars also have tiny toxic hairs. These hairs contain an irritant protein, can be carried by the wind and remain active in old nests for months.
Exposure can cause itchy skin rashes, eye irritation, sore throats and occasionally breathing difficulties and in rare cases, allergic reactions.
Pets and livestock are also at risk, particularly if they sniff or ingest the caterpillars.
When are they active?
There is one generation per year, with the greatest risk period from June to August, but nests should always be avoided because they can contain thousands of shed hairs.
How to identify them - look out for:
- processions of caterpillars moving nose-to-tail
- white, silken nests on oak trunks or branches (turning brown later in the season)
- leaves being eaten or stripped from oak trees
The following resource may help you confirm your identification: Native species that may be mistaken for oak processionary moth - Forest Research
Protect yourself
- keep your distance and teach children to stay away
- keep your pets away - caterpillars and nests can be on the ground, and animals can suffer considerable distress from hairs in their mouths and noses
- don't touch or attempt to remove them yourself
When submitting a report, please include photographs and location details.
Sightings can also also be reported via Tree Alert on-line or e-mail details to opm@forestrycommission.gov.uk Alternatively, you can call 0300 067 4442.
Visit Managing oak processionary moth in England - GOV.UK for more information.
The oak processionary moth is a powerful reminder that even small species can have wide-reaching consequences. By staying informed and reporting sightings, we can all play a role in protecting both our iconic oak trees and community wellbeing.