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NEWS RELEASE No: 1597419 JUNE 2013

‘Don’t touch’ warning over oak processionary caterpillar nests in London and Berkshire

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White, silken nest of oak processionary moth (Thaumetopoea processionea) on trunk of oak tree

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

They are also being alerted to protect animals from the nests, and to report sightings to the Forestry Commission or local Councils.

This is because the nests can contain thousands of the caterpillars’ hairs, which contain an irritating substance called thaumetopoein, which can cause painful skin rashes and, sometimes, eye and throat irritations in people and animals.
The caterpillars are the larval stages of the pest oak processionary moth (OPM), and June is the time of year when they build their distinctive white, silken, webbing nests on the trunks and branches of oak trees. They also make long, white trails of silken webbing on oak trees. They retreat into the nests between feeding sessions, and, later in the summer, to pupate into adult moths.

They 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 and caterpillars, but not to touch or approach them:

“The caterpillars are building nests in oak trees in preparation for pupating into adult moths in a few weeks’ time. We encourage anyone who sees them 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 oak 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 Yvonne Doyle, London Regional Director of Public Health England, endorsed the ‘don’t touch’ message.

“The nests can be full of irritating hairs, even after the moths have emerged, and the irritating substance in the hairs can remain active 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 111, 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 oak tree’s bark, and can be found anywhere in the oak tree from the trunk to the main branches. However, they can fall to the ground, where they can be accessible to children and inquisitive animals such as cats and dogs.

Guidance on identifying OPM is available on the Forestry Commission’s website at

  • Sighting reports – can be sent to the local council, or to the Forestry Commission using its Tree Alert app or on-line form. Council contacts and access to Tree Alert are both available at the above web address.
  • Health advice - Anyone who is worried by an intensely itchy or painful skin rash, sore throat or irritated eyes, and who might have been near oak trees infested with OPM, should consult their GP or NHS 111. Health information is also available from Public Health England at under ‘Oak Processionary Moth’. Anyone concerned about their pets should contact a vet.
  • Pest control - A list of local operators who can deal with OPM is available from the Forestry Commission on 0131 314 6414 /, or from local Councils.
  • Working on oak trees – Anyone having oak trees pruned or felled in 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.


  1. Affected areas - The London outbreaks of oak processionary moth affect, or could affect, oak trees in the following London local authority areas: Barnet, Brent, Bromley, Camden, City of London, City of Westminster, Croydon, Ealing, Elmbridge, Epsom & Ewell, Haringey, Hammersmith & Fulham, Harrow, Hillingdon, Hounslow, Islington, Kensington & Chelsea, Kingston Upon Thames, Lambeth, Merton, Richmond Upon Thames, Southwark, Spelthorne, Sutton and Wandsworth.
  2. The caterpillars began emerging from eggs in oak trees in late April, and we, local authorities and tree owners have been having affected oak trees treated to destroy as many of the pests as possible. The period during which any surviving caterpillars build nests in oak trees 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 peak danger period for human and animal health is from mid-May to the end of July.
  3. The Forestry Commission hopes that the smaller Pangbourne and Bromley/Croydon outbreaks can be eradicated. However, it has not proved possible to eradicate the larger London outbreak, first detected in oak trees in Ealing and Richmond in 2006. The objective there is to slow or prevent its spread, and keep its population as low as possible.
  4. Oak processionary moth (Thaumetopoea processionea) gets its name from the caterpillars' habit of moving about in nose-to-tail processions.
  5. The species usually pupates in the nests in late June and early July to emerge as adult 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.
  6. A native of southern and central Europe, it most likely came into Britain as over-wintering eggs previously laid on semi-mature oak trees imported for planting in landscaping projects. Although a native of southern Europe, OPM has become established as far north as The Netherlands over the past 20 years, aided by the trade in live plants. 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.
  7. OPM will occasionally attack other broadleaved trees such as hornbeam, hazel, beech, sweet chestnut and birch, but usually only where these trees are close to oak trees 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 – Peta Stoddart-Crompton, 01635 519670;
  • Public Health England - Tycie West or Cian Daly, 020 7811 7243/7242;;
  • Royal Botanic Gardens, Kew - Anna Quenby, Bronwyn Friedlander or Tarryn Barrowman, 020 8332 5607,;
  • The Royal Parks – Jessica Chambers, 0300 061 2128; ; and
  • The press offices of the London local authorities named above.