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Oak Processionary Moth – warning issued

Oak Processionary Moth Hertsmere’s parks and open spaces are a haven for residents during the summer but we’re asking visitors to be vigilant against a pest found in oak trees.

The Oak Processionary Moth (OPM) has been found in a number of parks and open spaces across the borough and residents are advised to avoid all contact with the caterpillar and their nest as it can cause itchy rashes, eye irritations and, in severe cases, breathing difficulties in people and pets.

Hertsmere’s parks team is working to manage Oak Processionary Moths in our area and will be putting up safety notices in those parks affected to warn the public. If you discover an OPM nest or caterpillar, we ask that you ‘spot it, avoid it, report it’.

This time of year is when the caterpillars emerge and feed before pupating into adult moths, but nests, even old ones, should not be touched at any time.

The nests are typically dome or teardrop-shaped, ranging in size – from a ping pong ball to as large as a rucksack. They are white when fresh, but soon become discoloured and brown. The caterpillars have black heads and bodies covered in long white hairs and are easily recognised by a distinctive habit of crawling in large groups, nose-to-tail forming long lines on or around oak trees.

If you see any oak processionary nests or caterpillars (OPM) in a council park you should report them immediately on 020 8207 2277 or email:

If they are on a privately owned oak tree, report them to the Forestry Commission using the Tree Alert online pest reporting form. If you cannot use Tree Alert, you may report them by telephone on 0300 067 4442 or by email at

For more information on the Oak Processionary Moth, including distribution maps and more detailed guidance on treatment visit: or