Oak processionary moth caterpillars
Jan 16 2018

Oak processionary moth

Last year Woodland Trust staff met with The National Trust at Osterley Park. The purpose of the meeting was to begin looking at how we manage the risk of OPM (Oak processionary moth) collectively.

Oak processionary moth (Thaumetopoea processionea, OPM) was first accidentally introduced to Britain in 2005, and it is theoretically possible that if it were to spread it could survive and breed in much of England and Wales. The larvae, or caterpillars, of OPM are a pest because they can affect the health of oak trees, people and animals.

OPM caterpillars are most easily recognised by their distinctive habit of moving about in late spring and early summer in nose-to-tail processions, from which they derive their name, and the fact that they live and feed almost exclusively on oak trees. They can sometimes be seen processing across the ground between oak trees, and clustering together as they feed on oak leaves.

The caterpillars have thousands of tiny hairs which contain an urticating, or irritating, substance. Contact with the hairs can cause itching, skin rashes and, less commonly, sore throats, breathing difficulties and eye problems.

This can happen if people or animals touch the caterpillars or their nests, or if the hairs are blown into contact by the wind. The caterpillars can also shed the hairs as a defence mechanism, and lots of hairs are left in the nests, which is why the nests should not be touched.

The nest may persist for years after the caterpillars have left them, and must always be handled/disposed of with caution.

The Woodland Trust are currently forming a working group with the National Trust, and, with other peer organizations to look at how best we manage the risk posed by these creatures. Topics to be discussed include:

  • Risk zoning and deciding when to act
  • Safety precautions for staff/volunteers/visitors         
  • Effective eradication & disposal

Obviously it’s very early days, but further information will be made available in time. For now Woodland Trust staff and volunteers in the south of England must be aware of the possible risk, and manage it accordingly.

The confirmed outbreaks of breeding OPM in Britain are as follows:

  • Several boroughs in West and South-West London and the Elmbridge and Spelthorne districts of Surrey
  • The Pangbourne area of West Berkshire
  • Bromley, Croydon and Lewisham Boroughs in South London
  • Hackney, Waltham Forest, Tower Hamlets and Newham Boroughs in East London
  • Guildford District in Surrey
  • A small number of nests have also been found in Watford, Hertfordshire

Health precautions

People in the affected areas can take these simple precautions to help minimise the health risks to themselves and their pets and livestock:


  • 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. Covering or stabling livestock can help
  • 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

Any sighting of OPM must be reported to the Forestry Commission – you can do so here:  http://www.forestry.gov.uk/treealert

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