Oak Processionary Moths [OPM]

Oak Processionary Moth - CaterpillarOak processionary moths (OPM) are a species whose caterpillars cause significant damage to oak trees by consuming their foliage, and can also pose a public health risk to both people and animals. 

Be alert for Oak Processionary Moth caterpillars when you are out and about in Surrey Heath. ​They are mainly present on Oaks but can also colonise other tree species. If you spot any do not touch.

Take a photo if possible, make a note of the location and report at trees@surreyheath.gov.uk - more information below.

The Forestry Commission has produced both a short leaflet and an in depth "OPM Manual".

As the name suggests, the caterpillars move in nose-to-tail processions along the main stems and branches of oak trees or on the ground and often cluster together. They build white, silken webbing trails and nests – usually dome or teardrop-shaped – on tree trunks and branches rather than amongst the leaves.

In the UK, OPM (scientific name Thaumetopoea processioneais) is now known to be present throughout the whole of London [Richmond Park currently spends £250k annually to contain their infestation] and many neighbouring boroughs and counties. A protein in the caterpillars' barbed hairs can cause severe skin and eye irritations, sore throats and breathing difficulties in people and animals who come into contact with them. The caterpillars pupate into adult moths in their nests on the trunks and branches of Oak trees from early July to early September and has resulted in a government-led programme of survey and management in these areas to minimise populations, spread and impacts on both trees and humans.

Oak Processionary Moth

To trees: OPM caterpillars can threaten the health of several species of oak trees (Quercus species) because they feed on the leaves. Large populations can defoliate, or strip bare, large parts of oak trees, leaving them vulnerable to attack by other pests and diseases, and less able to withstand stresses such as drought and flood. They will only feed on other trees if they run short of oak leaves to eat, and have been seen on Hornbeam, Hazel, Beech, Sweet Chestnut and Birch.

To people and animals: The caterpillars' thousands of barbed hairs contain an urticating, or irritating, substance called thaumetopoein. 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 nests should not be touched without protective clothing.

Forestry Commission and the NHS advise people in affected areas to take the following  precautions to minimise the health risks to themselves, their pets and livestock:

Do Not:

  • touch or approach nests or caterpillars;
  • let children touch or approach nests or caterpillars;
  • let animals touch or approach nests or caterpillars; or
  • 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 suspected OPM contact;
  • call NHS 111 or see a doctor if you think you or someone in your care has had a serious allergic reaction;
  • consult 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 Council Tree Officer on 01276 707100 or contact.centre@surreyheath.gov.uk or trees@surreyheath.gov.uk or the Forestry Commission using the Tree Alert on-line reporting form or e-mail opm@forestrycommission.gov.uk, or telephone 0300 067 4442.

Following your report of nests to Forest England you can engage specialist contractors to remove the nest and caterpillars at your own expense from your trees, these contractors can be found on the Arboricultural Association website.

Tree surgeons, forestry and ground-care workers, and others working on or close to oak trees in the affected areas, should wear full protective clothing, and familiarise themselves with the signs of OPM presence and the regulations applying to handling and moving oak material.

For more information visit the Forestry Commission website.