Residents of Pangbourne in Berkshire and parts of west London are being cautioned not to touch oak processionary moth caterpillars hatching in oak trees. They are also being asked to report sightings.
The caterpillars are a forestry pest and can be a health hazard. They damage oak trees by feeding on leaves and their tiny hairs contain a toxin that can cause skin rashes as well as eye and throat irritations.
Officials are dealing with outbreaks in five west London boroughs and in Pangbourne, West Berkshire.
Since the species was first found in Pangbourne in 2010, the Forestry Commission and West Berkshire Council have begun efforts to eradicate it.
It has also become established in the London Boroughs of Ealing, Brent, Hounslow, Richmond upon Thames and Hammersmith & Fulham, since it was found there in 2006. Eradication from this area is now considered impractical, so the Forestry Commission's policy is to work to contain it. The commission is therefore surveying a 10km buffer zone around the infested area.