St Albans residents are advised to watch out for moth larvae which can cause allergic reactions.
Oak processionary moths were recently recorded in Welham Green, near the boundary to St Albans.
Tracy Harvey, head of planning and building control at St Albans District Council, said: "Our trees and woodlands team has been monitoring the advance of oak processionary moths across the country for some months.
"Infestations have been confirmed in neighbouring districts and now we are looking into incidents reported recently in St Albans district to see if they can be confirmed.
"The larvae (caterpillars) are covered in irritating hairs that contain a toxin. Contact with these hairs, or their inhalation, can cause skin irritation and allergic reactions.
"We advise people not to touch the caterpillars, and to keep dogs away too."
Sightings should be reported at www.forestresearch.gov.uk/tools-and-resources/tree-alert.
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