Public warned of health risk after hazardous caterpillars found

Oak Processionary Moths

Oak Processionary Moths can cause itchy rashes, eye and throat irritations, while also posing a health risk to trees and animals. - Credit: The Forestry Commission

The public have been warned about a public health risk and told to avoid a hazardous caterpillar species after they were found near Cuffley.

Oak Processionary Moths has been found in Northaw Great Wood, with the insect known to cause health problems for oak trees, people and animals.

The pests can cause itchy rashes, eye and throat irritations, and should not be touched under any circumstances.

Visitors to the site have been asked to take extra care when walking in the woods and are advised to not touch or go near caterpillars or nests.

If they do come into contact with the caterpillars, they should call NHS 111 immediately.

After informing Welwyn Hatfield Borough Council of the caterpillars, The Forestry Commission will inspect, and if necessary, remove and dispose of the live and newly formed OPM nests by August 20.

Nests are typically dome or teardrop-shaped, and usually the size of a tennis ball. They are white when fresh, but soon become discoloured and brown.

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The caterpillars have black heads and bodies covered in long white hairs and crawl in large groups, nose-to-tail, forming long lines on or around oak trees.

“We would like to reassure the public that as long as the Oak Processionary Moths are left alone, there is no danger to the public,” said Durk Reyner, head of environment at WHBC.

“However, it would be helpful if visitors reported any sightings to the Forestry Commission via its Tree Alert online form.

“Reports help inform the management and treatment programme and enables the Forestry Commission to slow the spread of this pest.”

Oak Processionary Moth was first identified in London in 2006 after being accidentally imported, and has since spread to some surrounding counties.

The greatest risk period is May to July when the caterpillars emerge and feed before turning into adult moths.

Although most of Britain has pest free area status, meaning the pest is not known to be present in much of England, researches admitted in late 2021 that more needed to be done to control the OPM spread.

The public can also report sightings by email to, or by phone on 03000 674 442.