Forestry Commission logo

Nests removed and destroyed after oak processionary caterpillars found in Watford

This news story is now over a year old and information may no longer be accurate or up-to-date. It might also contain obsolete links.
Please use our search link on the left to look for more recent information.
L6 creating nest

Nests of tree pest oak processionary moth (OPM) caterpillars are being removed and destroyed in Watford after being found in the area this week.

People are being advised not to touch the caterpillars and nests, to keep animals away from the pests, and to report sightings to the Forestry Commission.

OPM can affect trees, people and animals, and was first discovered in England in London in 2005. Tree and public health authorities in Hertfordshire have been preparing for the possibility of its spreading into the county.

The caterpillars shed thousands of their tiny hairs in the nests, and these can cause itchy skin rashes and eye irritations. In extremely rare cases, they can cause breathing difficulties in people and animals. The caterpillars eat oak leaves, leaving infested trees weakened and vulnerable to other threats.

Steve Scott, the Forestry Commission’s East England Director, encouraged local people to help tackle the pest by reporting sightings of the nests and caterpillars, but not to touch or approach them:

“We want to keep our woods, parks and gardens safe for everyone to enjoy, so we are removing the nests immediately, and surveying oak trees in the surrounding area to check whether there are any more.

“The public can help us by reporting OPM nests and caterpillars to us so that they can be properly removed.

“However, please don’t try to remove the nests yourself. They need to be removed by people with the right training and equipment, and disposed of properly.”

Mr Scott added that protecting the country from plant and tree diseases is important for our economy, the environment and our health. The Government is committed to protecting our borders from pests and diseases and building the resilience of our trees and plants, and it has invested more than £21 million into tree health research.

Jim McManus, director of public health for Hertfordshire, said:

"The key risk to pets and humans is from the caterpillars' hairs, so we strongly support the 'don't touch or approach the nests or caterpillars’ advice from the Forestry Commission. People should also keep their pets and livestock away.

"We have issued advice to local GPs and health professionals and to accident and emergency departments to help them identify when patients have been affected by the caterpillars, and to advise them on treatment.”


  1. A map showing where OPM is known to be present is available at
  2. The caterpillars begin emerging from eggs in oak trees in March or April. They build nests in June and congregate in them between feeding sessions. They then pupate in the nests in July to emerge as adult moths, which lay their eggs in oak trees, and the caterpillars emerge from them the following spring. The white nests can be anywhere between ground level and many metres high, and become discoloured and harder to see after several days. They can fall out of trees, creating a hazard to curious children and pets, and grazing livestock.
  3. The peak risk period for human and animal health is from mid-May to the end of July. However, nests should not be approached at any time, because the hairs can remain active for a long period.
  4. OPM (Thaumetopoea processionea) gets its name from the caterpillars' habit of moving about in nose-to-tail processions. It gets the first part of its scientific name from thaumetopoein, the irritating protein in its hairs.
  5. OPM is native to southern Europe, where predators and local environmental factors keep its numbers in check. It was most likely accidentally introduced to Britain as over-wintering eggs previously laid on semi-mature oak trees imported for planting schemes. It has become established as far north as The Netherlands and northern Germany in recent years, aided by the movement of live plants in trade.
  6. OPM will occasionally attack other broadleaved trees such as hornbeam, hazel, beech, sweet chestnut and birch, but usually only where these trees are close to oak trees which they have stripped of leaves so that they are short of their preferred food.