Sightings of oak processionary moth caterpillars should be reported to local authorities and the Forestry Commission, after the pests started hatching at the Royal Botanic Gardens, Kew.
The botanic gardens is now leading an alert system for affected areas, to help get the problem under control at the earliest stage.
The affected boroughs are Ealing, Hounslow, Brent, Richmond Upon Thames and Hammersmith & Fulham.
Over the coming weeks surveyors will be looking for and mapping trees with caterpillars in them. They will be followed by specially equipped and trained pest control operators to deal with them. Later in the summer the operators will also remove and destroy nests made by any remaining caterpillars, which would pupate in the nests before re-emerging as moths.
Roddie Burgess, head of the Forestry Commission's Plant health Service, said: "We welcome reports of sightings 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, we strongly advise people not to touch or approach the caterpillars or their nests because of the health risks that the toxic hairs pose to them and their pets and livestock.
"Nor should they try to treat the caterpillars or remove the nests themselves. These jobs need to be carefully timed to be most effective, and are best done by specially trained operators."
A representative for RBG Kew explained: "We have been affected by oak processionary moth year on year and the whole purpose of the alert system is to let people know what stage we are at her at Kew so they can keep an eye out. Our lgardeners are probably more likely to spot something and that is why we are leading on [the scheme]."
Sighting reports and photographs can be sent to Forest Research, the scientific research arm of the Forestry Commission, on 01420 22255 or firstname.lastname@example.org, or to the local council.
Ealing: call 020 8825 5000 or e-mail email@example.com
Richmond upon Thames: call 08456 122660 or e-mail firstname.lastname@example.org
Brent: call 020 8937 5050 or e-mail email@example.com
Hounslow: call 020 8583 5555 or e-mail firstname.lastname@example.org
Hammersmith & Fulham: call 020 8753 3046 or email email@example.com.
Sightings in Richmond Park can be reported to the park office, 020 8948 3209; firstname.lastname@example.org.