People in parts of London and Berkshire are being reminded not to touch caterpillars of the oak processionary moth (OPM), which are beginning to emerge in oak trees in these areas.
They are also advised to keep children, pets and livestock away from the caterpillars and their nests, which can cause health problems, and to report any sightings.
Affected areas are several boroughs in West and South-West London, Bromley and Croydon in South London, Elmbridge in Surrey, and Pangbourne in Berkshire.
Ian Gambles, the Forestry Commission's director England, said: "We welcome reports of the caterpillars or their nests from the public or others, such as gardeners, tree surgeons and groundcare workers, who are out and about near oak trees.
"However, they should not try to remove the caterpillars or nests themselves. This needs to be carefully timed to be effective, and is most safely done by specially trained and equipped operators."
OPM caterpillars are a tree pest which was accidentally introduced to Britain. They feed on oak leaves, and in large numbers they can severely defoliate trees and leave them vulnerable to other pests and diseases.
Their tiny hairs contain a protein which can cause itchy skin rashes, eye and throat irritations and, occasionally, breathing difficulties, in people and animals.
The hairs can be blown on the wind, and left in their nests in and under oak trees. The greatest risk period is May to July, although nests should not be approached at any time.
Sighting reports can be sent to the local council, or to the Forestry Commission via an on-line form at www.forestry.gov.uk/opm
Comments