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NEWS RELEASE No: 1658316 MAY 2016

Alert for caterpillar pest in oak trees

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L6 creating nest

People in parts of London, northern Surrey and West Berkshire are being reminded to look out for and report caterpillars of the oak processionary moth (OPM), which could be damaging oak trees in the area.

They are also advised to keep away from the caterpillars and their nests, because their hairs can cause itching skin rashes and other health problems, and to report sightings to the Forestry Commission.

OPM is a tree pest which was accidentally introduced to England. They feed on oak leaves, and in large numbers they can severely defoliate trees and leave them vulnerable to other pests, diseases and drought.

Their tiny hairs contain a protein which can cause itchy skin rashes and, less frequently, eye and throat irritations and breathing difficulties in people and animals. The hairs can be blown on the wind, and left in their nests on and under oak trees. The greatest risk period is May to July, when the caterpillars are active, although nests should not be touched at any time.

The Forestry Commission, councils and land managers are tackling the pest with a carefully controlled programme of tree treatment and nest removal. Alison Field, the Commission's South-East England Director, said the public could play an important role in helping to control the pest by reporting sightings.

“We need reports of the caterpillars or their nests from the public or others, such as gardeners, tree surgeons and ground-care workers, who work or relax near oak trees,” Ms Field said.

“However, they should not try to remove the caterpillars or nests themselves. This needs to be carefully timed to be effective, and is most safely done by specially trained and equipped pest control experts.”

Dr Deborah Turbitt, London Deputy Director for Health Protection for Public Health England, endorsed the ‘don’t touch’ advice, saying:

“We strongly advise people not to touch or approach the caterpillars or their nests because of the health risks posed by the hairs. Pets and livestock can also be affected, and should be kept away as well. The Forestry Commission website has pictures to help identify the pest.

“See a pharmacist for relief from milder skin or eye irritations following possible OPM contact, or consult a GP or NHS111 for more-serious reactions. Contact a vet if animals are more seriously affected.

“We have issued advice to local GPs and health professionals to help them identify when patients have been affected by the caterpillars, and to advise them on treatment.”

Infested oak trees should be treated by qualified operators under strict controls to ensure it is safe for people, pets, livestock and the environment.

  • Report OPM sightings to the Forestry Commission, preferably with its Tree Alert on-line form available from If you cannot use Tree Alert, email or 'phone reports to or 0300 067 4442.
  • Health advice is available from the “Insects that bite or sting” area of the NHS Choices website,
  • Anyone pruning or felling oak trees in the affected areas should contact Forestry Commission England’s Tree Health Unit beforehand on or 0300 067 4442 for advice about safe removal of the material.
  • Further information is available from

Notes to editor:

  1. A distribution map is available at The affected areas are divided into ‘core’, ‘control' and 'protected' zones. Areas around the outer boundaries of the outbreak areas form the ‘control and protected zones’, where the Forestry Commission will undertake control on private properties to prevent or minimise spread of the pest into new areas, funded by the Department for the Environment, Food & Rural Affairs (Defra). Control in the core zone is at property owners’ discretion and expense. Maps of the zones are available in the OPM manual at
  2. Sighting reports should include a precise description of the location. An Ordnance Survey grid reference is ideal, otherwise an accurate postal address with the full postcode, and/or a clear description of the tree's exact position, is helpful. Clear photographs are welcome. OPM lives almost exclusively in oak trees.
  3. OPM is tackled in the spring by treating oak trees with insecticide after the caterpillars have hatched, and in the summer by removing their silken webbing nests from the trunks and branches of oak trees. (They do not make nests among the leaves). These are typically dome- or teardrop-shaped, averaging the size of a tennis ball, although they can be much larger or smaller, before collapsing into a bag-like shape. They are white when fresh, becoming discoloured quite quickly and more difficult to see as a result.
  4. The products used to treat infested trees are safe for people, pets, livestock and the environment when used correctly. They are licensed by the Chemical Regulations Directorate, and applied by trained operators who are fully compliant with all the relevant and very strict health, safety and environmental protection requirements.
  5. OPM (scientific name Thaumetopoea processionea) gets its name from the caterpillars' habit of moving about in nose-to-tail processions. It gets the first part of its scientific name from thaumetopoein, the irritating protein in the caterpillars’ hairs.
  6. A native of southern Europe, OPM has become established as far north as The Netherlands in recent years. It most likely entered Britain as eggs laid on young oak trees before they were imported from mainland Europe for planting.
  7. The moths lay their eggs in oak trees in July and August, and the caterpillars emerge the following spring. They feed in groups, and congregate in their nests at other times. The caterpillars pupate in their nests in June and July and emerge as moths up to four weeks later.
  8. OPM will attack other trees such as hornbeam, hazel, beech, sweet chestnut and birch, but usually only when it is short of oak leaves.


  • Forestry Commission - Charlton Clark, 0300 067 5049;
  • Public Health England - Cian Daly, 020 7811 7242.