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NEWS RELEASE No: 1549612 JUNE 2012

Alert over oak processionary moth nests in London and Berkshire

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Nest of oak processionary moth

Residents and visitors to south and west London and Pangbourne in Berkshire are being reminded not to touch the nests which oak processionary moth caterpillars are now building in oak trees in these areas.

The caterpillars, the larval stages of the pest oak processionary moth (OPM), leave thousands of poisonous hairs in the white, silken webbing nests that they are now building on the trunks and branches of oak trees. These tiny hairs contain a toxin called thaumetopoein, which can cause painful rashes and eye and throat irritations in people and animals who come into contact with them.

The caterpillars are also a tree pest because they eat oak leaves, and large populations can strip trees of their leaves, leaving them weakened and vulnerable to other threats. Alison Field, South East England Director for the Forestry Commission, encouraged local people to help tackle the pest by reporting sightings of the nests, but not to touch or approach them:

“The caterpillars are building nests in preparation for pupating into adult moths in a few weeks’ time. We encourage anyone who sees nests or caterpillars to report them to us or to their local council so that they can be dealt with properly.

“We also advise people against trying to remove the nests themselves, even if they own the tree. To be as effective and safe as possible this job needs to be timed just right and done by people with the right training and equipment, and the nests must be disposed of properly.”

Dr Brian McCloskey, Director of the Health Protection Agency (HPA) in London, endorsed the ‘don’t touch’ message.

“The nests can be full of toxic hairs, even after the moths have emerged, and the hairs remain toxic for a long time. The hairs can be blown about by the wind, so it’s important that people and animals do not touch or go near the nests.

“Anyone who experiences an itchy or painful skin rash or a sore throat and irritated eyes after being near oak trees in these areas should consult their GP or NHS Direct, who have been given advice about recognising the symptoms and appropriate treatment.”

The nests are typically about the size of a tennis ball. They are usually white when new, but become discoloured to match the colour of the tree’s bark, and can be found anywhere in the tree from the trunk to the main branches. However, they can fall to the ground after being dislodged by factors such as wind and rain, where they can be accessible to inquisitive children and animals such as cats and dogs.

The caterpillars began emerging from eggs a few weeks ago, and local authorities and tree owners have been having affected trees treated to destroy as many of the pests as possible. The period during which any surviving caterpillars build nests is the last opportunity of the year to reduce the population, especially if the nests can be removed while the pupae (the transitional lifecycle stage between caterpillar and moth) are inside them.

The NHS Direct number is 0845 4647, and health information is available from the HPA website, Further information about OPM is available from the Forestry Commission website at


  1. The Forestry Commission’s Plant Health Service can provide details of pest removal services which are qualified and equipped to deal with OPM. Call 0131 314 6414 or email . Local councils’ trees and parks services can also provide these details.
  2. People who are having oak trees pruned or felled in any of the affected areas must contact the Forestry Commission’s Plant Health Service beforehand on or 0131 314 6414 for advice about safe removal of the material.
  3. Sighting reports can be sent to the Forestry Commission’s Forest Research agency at or 01420 22255, or to the local council. Sightings in one of the affected Royal Parks such as Richmond or Bushy Park can be made to Royal Parks staff. 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. Digital photographs may also be sent to aid identification.
  4. The Forestry Commission hopes that the small Pangbourne outbreak can be eradicated. However, it has not proved possible to eradicate the larger London outbreak, first detected in Ealing and Richmond in 2006. The objective there is to slow or prevent its spread, and keep its population as low as possible.
  5. The London outbreak area is divided into two zones – the core outbreak area comprising parts of Brent, Ealing, Hounslow, Richmond Upon Thames and Hammersmith & Fulham Boroughs, where infestations have been confirmed in this or past years; and a 10-kilometre (6-mile) ‘buffer’ zone around the core area, taking in areas of Wandsworth (where infestations have been found this year), Barnet, Harrow, Hillingdon, Camden, Westminster, Kensington & Chelsea, Lambeth, Merton, Kingston Upon Thames, Sutton, Croydon and Elmbridge, Epsom and Ewell, Southwark, City of London, Islington, Haringey and Spelthorne. Forestry Commission inspectors are surveying the buffer zone for infestations. They will serve statutory Plant Health Notices on the owners of any infested trees in the buffer zone, and in the core zone where the infestation threatens to spread into the buffer zone, requiring them to have the infestations removed. Management of infestations in the core zone is primarily the responsibility of local authorities and tree owners.
  6. Oak processionary moth (Thaumetopoea processionea) gets its name from the caterpillars' habit of moving about in nose-to-tail processions. A native of southern and central Europe, it was first identified on oak trees in west London in 2006. It most likely came into Britain as over-wintering eggs previously laid on semi-mature trees imported for planting in landscaping projects.
  7. The caterpillars and their nests are always dangerous to approach because of the presence of toxic hairs, which can remain present and harmful in old nests for some years. However, the peak danger period for human health is from mid-May to the end of July.
  8. The species usually pupates in the nests in late June and early July and emerge as moths between one and four weeks later. The moths lay their eggs in oak trees in July and August, and the eggs hatch caterpillars the following spring. The caterpillars feed in groups, and at other times congregate in the communal nests.
  9. Although a native of southern Europe, OPM has become established as far north as The Netherlands over the past 20 years. Climate change might have been a factor: fewer late spring frosts and lower spring rainfall, which could kill emerging caterpillars, might have improved its chances of survival in more northern latitudes.
  10. OPM will attack other broadleaved trees such as hornbeam, hazel, beech, sweet chestnut and birch, but usually only where these trees are close to oaks which they have stripped of leaves so that they are short of their preferred food.


  • Forestry Commission / Forest Research - Charlton Clark, 0131 314 6500 or 07810 181067;
  • West Berkshire Council – Arthur Cullen, 01635 519675;
  • Health Protection Agency - Tycie West or Nikki Karpeles, 020 7811 7243/7242;
  • Royal Botanic Gardens, Kew - Anna Quenby, Bronwyn Friedlander or Tarryn Barrowman, 020 8332 5607,;
  • The Royal Parks – Joanna Hughes, 0300 061 2128; ; and
  • The press offices of the London local authorities named above.