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Date updated: 11/06/2024

The oak processionary moth (thaumetopoea processionea; OPM) is a pest which has established itself in oak trees in parts of the UK in recent years after being accidentally introduced from mainland Europe.

The caterpillars, which emerge about April every year have tiny hairs which contain an irritating substance called thaumetopoein. This substance can cause itching skin rashes, eye irritations and sore throats in people and animals who come into contact with them. In rare cases they can cause breathing difficulties and severe allergic reactions.

If you do have a serious allergic reaction, call NHS 111 or see a doctor. Similarly, consult a vet for badly affected animals.

Oak processionary moth caterpillars
Oak processionary moth caterpillars

What to do if you see OPM

If you see any OPM nests or caterpillars, do not touch or approach them. The caterpillars have a distinctive habit of moving about in or under oak trees in nose-to-tail processions, which gives them their name. The nests are white when new, but quickly become discoloured and harder to see.

The main risk period is April to July, when the caterpillars are active. However, avoid nests, even 'spent' nests, at any time, because the hairs in them can remain irritating for several months.

How to report OPM

If you see any OPM nests or caterpillars please report them to the main office of the green space you are visiting.

  • Burnham Beeches and Stoke Common: 01753 647358
  • The Commons: 01372 279083
  • City Gardens: 020 7374 4127
  • City of London Cemetery and Crematorium: 020 8530 2151
  • Epping Forest: 020 8532 1010
  • Hampstead Heath: 020 7332 3322
  • Highgate Wood: 020 8444 6129
  • Queen's Park: 020 8969 5661
  • West Ham Park: 020 8472 3584

Note that OPM does not live on walls, fences or other structures, so there is no need to report caterpillars or nests in these situations.

They also only attack other trees if they run short of oak leaves to feed on, so please do not report caterpillars or nests in other trees unless there are stripped oak trees nearby.

Control programme

We are promoting a nature-based solution to management of this pest by encouraging natural predation by birds and parasitism by insects. 

Further information

For detailed information about OPM, including videos, a guide to recognising it, a leaflet and poster, maps showing where it is known to be present, and guidance for tree surgeons and people who own oak trees in the affected areas, visit the Forest Research website.