The public has been warned to keep an eye out for a pest which poses a health risk to people.

The oak processionary moth caterpillar damages trees as well as causing itchy rashes, eye and throat irritation.

The warning comes at the start of the greatest risk period for the non-native species, which is predominately found in south east England having been accidentally transported from Europe in trees for planting.

The caterpillars descend oak trees in a head-to-tail processionary form, and feed on the tree’s leaves as they migrate, harming their growth and weakening the tree, leaving it vulnerable to other stresses such as drought and disease, experts said.

The Oldham Times: Oak processionary moth caterpillars Oak processionary moth caterpillars (Image: Alamy/PA)

Oak processionary moth caterpillars and their nests of white silken webbing contain hairs that can cause itchy rashes, eye and throat irritation and should not be touched under any circumstances, they warned.

The public are also urged never to try and dispose of the nests, found in the trunk or branches of oak trees, themselves.

The warning has been issued by the Forestry Commission, which is urging the public to report any sightings of the oak processionary moth caterpillars to them.

Andrew Hoppit, oak processionary moth project manager, said:  “It is important those living and working in areas affected by oak processionary moth remain vigilant about the health risks they pose, when enjoying outdoor spaces, as we are entering the greatest risk period.”

The insect was first identified in London in 2006, and has since spread to surrounding counties in the South East.

Professor Nicola Spence, UK chief plant health officer, said: “Oak trees are an iconic and much-loved part of our British landscape.

“By reporting any sightings of the oak processionary moth to the Forestry Commission, we can all minimise the pest’s spread as well as reduce their impact on tree health.

“I would advise that members of the public living in London, the surrounding areas and Derbyshire, avoid any contact with the caterpillar and its nests, as this can cause irritation.”