People in parts of London and Berkshire are being reminded 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.
The affected areas are several boroughs in West and South-West London, Bromley and Croydon in South London, and Pangbourne in West Berkshire.
Caterpillars of the oak processionary moth (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.
They are a tree pest because they damage oak trees by feeding on the leaves, in some cases leaving the trees severely defoliated and vulnerable to other pests, diseases or drought.
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.
Forestry Commission England director Ian Gambles said: ""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.
"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."
Public Health England London regional director Dr Yvonne Doyle added: "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. If you would like to know what these caterpillars look like, please see the Forestry Commission’s website for pictures.
"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.
"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.
Sightings can be sent to the local council, or to the Forestry Commission, using the Commission’s Tree Alert app or on-line form at www.forestry.gov.uk/oakprocessionarymoth.
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 (www.hpa.org.uk) under ‘Oak Processionary Moth’.
Pest control - A list of local operators who can deal with OPM is available from the Forestry Commission on 0131 314 6414 / firstname.lastname@example.org, 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 on email@example.com or 0131 314 6414 for advice about safe removal of the material.
Further information is available from www.forestry.gov.uk/oakprocessionarymoth.