Beware of Oak Processionary Moth

a Pest to Trees, Humans and Animals

As a landscaping company, this is a subject close to the heart and one which we feel compelled to share with our readers.

You may not be familiar with the oak processionary moth (OPM) but this is a very real and present danger in the UK.

The caterpillars (larvae) of the OPM are not only pests of oak trees but also a hazard to human and animal health. Now present throughout much of Greater London and some surrounding counties, OPM was accidentally introduced to England in 2005. It’s now subject to a government-led programme of survey and control, aimed at minimising the OPM population, spread and impact.



Although OPM is a native of central and southern Europe, over the past 20 years, its range has been expanding northwards. This has been exacerbated by the trade in live oak trees and possibly by a warming climate.

If it continues to spread, OPM might eventually colonise many other parts of England and Wales.


The threat to oak trees

OPM is a pest to oak trees because its caterpillars feed on the leaves of several species of oak. Large OPM populations can strip whole oak trees bare, leaving them more vulnerable to other pests and diseases and to other stresses, such as drought. Treating a diseased tree can require radical steps – felling and burning in the worst cases.


The threat to people and animals

It isn’t just trees that can be affected. The tiny hairs of older caterpillars contain an irritating protein called thaumetopoein. Human contact with these hairs can cause various allergic reactions in people and animals including itching skin rashes, eye irritations, sore throats and breathing difficulties. The risk of exposure to these hairs is highest in May and June.

The caterpillar hairs can be hard to detect and spread far and wide. The caterpillars shed them when threaten

ed, or they can be blown by the wind and accumulate in their nests, which can then fall to the ground. But the hairs are also prone to sticking to tree trunks, branches, grass and clothing as well as equipment, such as ropes used by tree surgeons and forestry workers.

Those most vulnerable to the health risks are curious children and pets, anyone spending time or working close to an infested tree, or grazing livestock and wild animals.


How do you spot the moths and caterpillars?

Whilst the moths themselves are an indistinct brown colour, similar to other species and consequently tricky to identify, they are no threat to health – it’s the caterpillars which are the problem.

  • They have a grey body and dark head. Older larvae have a central dark stripe with paler lines down each side;
  • They have very long, white hairs in contrast to the much shorter, almost undetectable irritating hairs;
  • They have a distinctive habit of moving about in late spring and early summer in nose-to-tail processions, hence the name. The processions are often arrow-headed, with one leader and subsequentrows of several caterpillars abreast;
  • Living and feeding almost exclusively on oak trees, they can sometimes be seen proceeding across the ground between oak trees;
  • They have also been observed feeding on other tree species – sweet chestnut, hazel, beech, birch and hornbeam;
  • They are only mainly seen in mid- to late spring and early summer (May, June and July);
  • They are not usually found on fences, walls and similar structures, such as garden furniture.

Other types of caterpillar, however, can easily be mistaken for OPM.



OPMs build their nests in early summer, typically on the trunks and branches of oak trees, occurring anywhere from ground level to high up in the tree, but almost never amongst the leaves. The nests can range in size from a few centimetres wide to stretching several feet up the trunk. Multiple nests can occur on the same tree or branch.

Made from distinctive, white, silken webbing, accompanied by white, silken trails on trunks and branches, after a short time, the nests become discoloured, making them more difficult to spot as a result.

They also occur in a variety of shapes, including hemispherical (half a ball), tear-drop shaped, bag-like and like a blanket that’s been stretched around part of an oak trunk or branch.

It doesn’t take much for the nests to fall out of trees and they are often found on the ground or they can remain attached to a tree for months at a time, after the larvae have pupated and the adult moths have emerged.


How to minimise health risks – do not:
  • touch or approach OPM nests or caterpillars;
  • let children or animals touch or approach nests or caterpillars;
  • attempt to remove nests or caterpillars yourself.


  • teach children not to touch or approach the nests or caterpillars;
  • train or restrain pets from touching or approaching them;
  • keep horses and livestock a safe distance from infested oak trees -covering or stabling can help;
  • see a pharmacist for relief from skin or eye irritations if you have suspected OPM contact;
  • call NHS111 or see a doctor if you think you or someone in your care has had a serious allergic reaction – tell the doctor you suspect OPM contact;
  • consult a veterinary surgeon if you think your pet or livestock have been seriously affected;
  • call in an arborist or pest control expert with relevant expertise to remove infestations in your own trees. Your local council, the Arboricultural Association, the British Pest Control Association or the Forestry Commission can provide a list of suitable operators in your area; and
  • anyone working on or near oak trees in the affected areas, should wear full protective clothing. The Oak Tree Owners’ OPM Manual has occupational health guidance.


To Report a Find

It’s important to learn how to recognise OPM nests and caterpillars and to protect yourself or children and animals in your care from the associated health hazards.

‘Spot it, avoid it, report it’ is the mantra the Government is encouraging everyone in affected areas to adopt.

If you think you’ve spotted a diseased tree, contact the Forestry Commission using the Tree Alert reporting tool. They will require three good-quality digital photographs to aid identification.

To contact the Forestry Commission’s OPM team:

To report a sighting of OPM:

Finally, oak trees and woodlands are wonderful places to visit and explore. Don’t be deterred from getting out and about and enjoying nature. Just stay alert and you’ll be fine.