Toxic caterpillars - what you need to know about the oak processionary moth

A species of oak-munching moth whose furry caterpillars can trigger rashes in humans and cause breathing difficulties in pets is invading England.

Watch out for the furry caterpillars creeping down an oak tree near you soon.

An invasion of oak processionary moths is under way, according to the Forestry Commission, with the moth’s larvae emerging from silken webbing nests in the branches of oak trees .

The larvae that survive insecticide treatment will grow big enough and crawl low enough down the trees to be spotted in parts of south-east England and London, but while the furry caterpillars look fascinating as they swarm in groups around oak trees, their hair can have a nasty effect on anyone who touches them.

“May is also about the time when they develop the hairs which contain a substance which can cause itching skin rashes, eye irritations and, occasionally, sore throats and breathing difficulties in people and animals that come into contact with them,” the Forestry Commission warns.

[Read more: Clothes moths are on their way: 10 ways to moth-proof your clothes and wardrobe]

What are oak processionary moths?

Thaumetopoea processionea or the oak processionary moth (OPM) is an invasive species, originally from southern Europe, which was accidentally introduced to British shores in 2005. It’s been moving further north in the last 20 years thanks to the movement of plants.

OPM caterpillars or larvae are deemed a pest because they’re a health hazard to humans, animals and oak trees. 

They’re easy to spot in spring and summer thanks to their habit of moving in a nose-to-tail procession (hence their name), but they exclusively live and feed on oak trees, stripping them bare of leaves.

Each caterpillar has around 63,000 distinctive, long white hairs, which contain the irritating substance called thaumetopoein.

How to stay safe

Do not

  • Touch or approach nests or caterpillars
  • Let children touch or approach nests or caterpillars
  • Let animals touch or approach nests or caterpillars
  • Try removing nests or caterpillars yourself


  • Teach children not to touch or approach the nests or caterpillars
  • Train or restrain pets from touching or approaching them
  • Keep horses and livestock a safe distance from infested oak trees
  • See a pharmacist for relief from skin or eye irritations after possible OPM contact
  • Call NHS111 or see a doctor if you think you or someone you care for has had a serious allergic reaction
  • See a vet if you think your pet or livestock has been seriously affected
  • Call in a pest control expert to remove infestations in your own trees; and report any sightings to the Forestry Commission.

More from BT