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Alert over caterpillar pest in London and Berkshire

| June 18, 2013

Caterpillars of the Oak Processionary Moth, Thaumetopoea processionea (OPM). Image credit: Forestry Commission (Crown copyright)

People in parts of London and South East England are being reminded this Spring by the Forestry Commission, Public Health England and local authorities not to touch caterpillars of the Oak Processionary Moth, which are beginning to emerge in oak trees in these locations. They are also advised to keep children, pets and livestock away from the caterpillars and their nests, and to report any sightings.

Caterpillars of the Oak Processionary Moth, Thaumetopoea processionea (OPM), which was accidentally introduced to Britain from mainland Europe in 2006, are a tree pest and pose a threat to human and animal health. The caterpillars damage oak trees by feeding on the leaves, in some cases leaving the trees severely defoliated and vulnerable to other pests, diseases or drought.

A native of southern Europe, OPM has become established as far north as The Netherlands over the past 20 years. It most likely entered Britain as eggs laid on young oak trees before they were imported from mainland Europe for planting here. The caterpillars pupate in their nests in late June and early July and emerge as moths up to four weeks later.

The moths lay their eggs in oak trees in July and August, and the caterpillars emerge from the eggs the following spring. The caterpillars feed in groups and at other times congregate in nests of matted, white, silken webbing, which are typically about the size of a tennis ball. OPM will attack other trees such as hornbeam, hazel, beech, sweet chestnut and birch, but usually only where oaks are severely defoliated and its preferred food of oak leaves is limited


Rash caused by contact with the hairs of OPM caterpillars. Image credit: Henry Kuppen.

They pose a risk to human and animal health because they have tiny, toxin-containing hairs which, on contact, can cause itchy skin rashes in people and animals. Eye and throat irritations have also been reported as symptoms. The hairs can be blown on the wind, and left in the silken, web-like nests which the caterpillars build in oak trees. They pose the greatest risk from May to July, although nests should not be approached at any time.

The Forestry Commission is working with local authorities and land managers to tackle the outbreaks with a carefully controlled programme of tree spraying and nest removal. Ian Gambles, Director of Forestry Commission England, said the public could help, but urged caution. “We need, and welcome, reports of the caterpillars or their nests from the public or others, such as gardeners and tree surgeons, who are out and about in areas with oak trees,” he said. “However, the public should not try to remove the caterpillars or nests themselves. This task needs to be carefully timed to be most effective, and is best done by specially trained and equipped operators.”

Dr Yvonne Doyle, London Regional Director of Public Health England, endorsed this advice, saying: “We strongly advise people not to touch or approach the caterpillars or their nests because of the health risks posed by the toxin-containing hairs. Pets can also be affected and should be kept away as well. “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 doctor or contact the NHS. “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 appropriate treatment.” Tree spraying is done by fully qualified operators under strict health, safety and environmental controls to ensure it is safe for humans and animals.

  • Sighting reports – can be sent to the local council, or to the Forestry Commission, using the Commission’s Tree Alert app or on-line form at
  • 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 the Public Health England website ( 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 at, or the local Council.
  • 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

Further information is available from

Published in International Pest Control – May/June 2013 issue.

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Category: Forestry-Plantation

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