Children and animals are being warned away from Ealing’s hairy caterpillars and their nests because their hairs can cause itching skin rashes and other health problems.
The caterpillars have tiny hairs which contain a protein that can cause itchy skin rashes and, less frequently, eye and throat irritations and breathing difficulties in people and animals.
The greatest risk period is May to July, although nests should not be approached at any time as detached hairs remain within them.
The Forestry Commission website has pictures to help identify the pest.
Ealing Council is working with the Forestry Commission to tackle the nests and moth population in its parks and open spaces with a carefully controlled programme of tree treatment and nest removal.
Householders who find evidence of the pest in their own oak trees should report it to the Forestry Commission.
Cabinet member for transport, environment and leisure, Bassam Mahfouz said: “We are urging residents to be vigilant at home, in schools and any outdoor spaces where there might be oak trees.
"We know this is the time of year when the caterpillars build their distinctive white, silken nests and trails on the trunks and branches of oak trees, and can sometimes be as big as a tennis ball.
“They become discoloured and harder to see after a time. They may be interesting to a curious child or pet, so please take care and report any sightings to the Forestry Commission.”
Ian Gambles, the Forestry Commission’s director for England, said: “The public can play an important role in helping to control the pest by reporting sightings.
“We need reports of the caterpillars or their nests from the public or others, such as gardeners, tree surgeons and ground-care workers, who work or relax 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.”
The Forestry Commission advises that the oak processionary caterpillars only build their nests on the trunks and branches of oak trees. They are never found among the leaves of an oak tree or on structures such as buildings, gates and fences, so caterpillars or nests in these situations need not be reported.
They are only rarely found in other trees, and usually only if they run out of oak leaves to eat.
Anyone displaying symptoms including skin or eye irritations following possible OPM contact should visit a pharmacist, or call NHS 111 for more-serious reactions.
Contact a vet if an animal is affected.
Sightings should be reported to the Forestry Commission, preferably using its on-line Tree Alert form available at www.forestry.gov.uk/opm. Alternatively, residents can email email@example.com or call 0300 067 4442.