The Forestry Commission has today (Thursday, May 11) urged the public to report sightings of oak processionary moth (OPM) caterpillars.

The forestry regulator said the greatest risk period for the tree pest is between June and August, as the caterpillars emerge to feed before turning into adult moths.

The oak processionary moth, first identified in London in 2006, has since spread to some surrounding counties in the South East.

Oak processionary moth caterpillars feed on the leaves of several species of oak tress and can cause the trees to lose their leaves, negatively impacting their growth and leaving them more vulnerable to other stresses like drought.

In humans, the caterpillars and their nests contain hairs which can cause itchy rashes and eye and throat irritations. The Forestry Commission said they should not be touched under any circumstances.

Oak processionary moth project manager, Andrew Hoppit, said: “As we enter the greatest risk period for oak processionary moth, it’s important that those living in affected areas understand the health risks so that they can be vigilant when enjoying outdoor spaces.

“The Forestry Commission has a whole host of information online that can be used to identify the moth, simply visit managing oak processionary moth in England.

“If you spot the pest, report the sighting via our TreeAlert portal.”

OPM and oak health

A UK government programme to manage OPM has been in place since 2012 and has seen £10 million invested into oak health over the last five years.

The investment targeted the management of OPM as well as research to develop novel control techniques and new policy responses.

In March of this year, the Forestry Commission updated operational zones and boundaries in a bid to manage the spread of the pest.

Prof. Nicola Spence, UK Chief Plant Health Officer, said: “Our oak trees are an iconic part of our British landscape.

“Reporting any sightings of oak processionary moth to the Forestry Commission will both minimise the pest’s spread and reduce the damaging impact it poses to tree health.

“The caterpillars and their nests can also cause irritation when touched by members of the public. As such, I would advise that those living in London and the surrounding areas avoid the pest.”

OPM nests are typically dome or teardrop-shaped, the Forestry Commission said, averaging the size of a tennis ball.

The are white when fresh, but soon become discoloured and brown. The caterpillars have black heads and bodies covered in long white hairs.