Oak Processionary Moth (Thaumetopoea processionea)

Oak Processionary Moth (OPM) caterpillars cause significant damage to oak trees and can pose risks to human and animal health.

Origin and History

OPM is a native of central and southern Europe, where predators and environmental and ecological factors usually keep its numbers in check and minimise its impact. However, its range has been expanding northwards over the past 20 years. The expansion has been aided by the movement of live oak trees in trade which might have OPM present on them, and perhaps also by a warming climate. It is now established as far north as The Netherlands and northern Germany, and has occasionally been seen in Sweden.

OPM was first accidentally discovered in Britain, in West London, in 2006. The introduction probably occurred during the autumn of 2005, the winter of 2005-06, or the spring of 2006, and almost certainly as eggs which had been laid on live oak plants before they were imported from continental Europe. The adults which emerged in 2006 laid eggs in nearby oak trees, establishing a local breeding population.

The current distribution of the pest has arisen from that and a small number of subsequent, similar introductions, and by natural spread from the original points of introduction. If it continues to spread, it might eventually colonise many other parts of England and Wales.


OPM is an established pest in London and surrounding areas, but the majority of the UK including Northern Ireland is designated as a Protected Zone and free from the pest.


OPM is a tree pest because its caterpillars feed on the leaves of several species of oak trees. Large populations can strip whole oak trees bare, leaving them more vulnerable to other pests and diseases, and to other stresses, such as drought. The picture below shows branches of an oak tree after heavy feeding by OPM caterpillars.

Oak tree following defoliation by OPM
The threat to people and animals

Older caterpillars develop tiny hairs containing an urticating, or irritating, protein called thaumetopoein. Contact with the hairs can cause itching skin rashes (pictured below) and eye irritations, sore throats, breathing difficulties and, rarely, allergic reactions in people and animals. The risk of exposure to these hairs is highest in May and June. 

Skin irritation caused by OPM
The caterpillars can shed the hairs when threatened or disturbed. The hairs can be blown by the wind, and they accumulate in the caterpillars’ nests, which can fall to the ground. They can stick to trunks, branches, grass and clothing as well as tree surgeons’, forestry and ground-care workers’ equipment, such as ropes.

Among the groups most vulnerable to the health hazards are:

  • curious children and pets;
  • people who work on or close to oak trees;
  • anyone spending time close to infested trees; and
  • grazing and browsing livestock and wild animals.



OPM moths, the adult form of the species, are undistinctive brown moths similar to other species, and are difficult to accurately identify. They are not a health hazard, and we do not require reports of sightings.


  • have a distinctive habit of moving about in late spring and early summer in nose-to-tail processions, from which they derive their name. The processions are often arrow-headed, with one leader and subsequent rows of several caterpillars abreast;
  • live and feed almost exclusively on oak trees. They can sometimes be seen processing across the ground between oak trees;
  • will usually only affect other broad-leaved tree species if they run short of oak leaves to eat - they have been observed feeding on sweet chestnut, hazel, beech, birch and hornbeam. However, they generally cannot complete their development on other tree species;
  • cluster together while they are feeding on oak leaves and moving from place to place;
  • are only seen in mid- to late spring and early summer (May, June and July)
  • have very long, white hairs which contrast markedly with the much shorter, almost undetectable irritating hairs;
  • have a grey body and dark head. Older larvae have a central dark stripe with paler lines down each side; and
  • are not usually found on fences, walls and similar structures, such as garden furniture.


OPM nest

  • are built in early summer;
  • are made on the trunks and branches (pictured) of oak trees;
  • are almost never made among the leaves of oak trees, or on any other tree or shrub species, or on fences, walls and similar structures. Such nests are usually made by harmless insect species, and need not be reported;
  • are made of distinctive, white, silken webbing, which are accompanied by white, silken trails on the trunks and branches of oak trees;
  • become discoloured after a short time, and more difficult to see as a result, as do the silken trails;
  • occur in a range of shapes, including hemispherical (half a ball), tear-drop shaped, bag-like, and like a blanket stretched around part of an oak trunk or branch;
  • range in size from a few centimetres wide to stretching several feet up the trunk;
  • can occur anywhere from ground level to high in the oak tree;
  • can fall out of oak trees and be found on the ground; and
  • can remain attached to the trees for many months after the larvae have pupated and the adult moths have emerged.

Several nests can occur on the same tree or branch.

Older caterpillars feed mainly at night and rest up in their nests during the day. Later in the summer they retreat completely into the nests as pupae, re-emerging a few weeks later as adult moths.

Health precautions

To minimise the health risks:

  • do not touch or approach OPM nests or caterpillars;
  • do not let children or animals touch or approach nests or caterpillars; and
  • do not try removing 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 after suspected OPM contact;
  • 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 has been seriously affected - tell the vet you suspect OPM contact;

if you work on or near oak trees in the affected areas, for example, as a tree surgeon or forestry, landscaping or ground-care worker, wear full protective clothing. 

Report a sighting

If you believe you have found an OPM nest or caterpillar, please report it immediately to us.

Use our Treecheck page to report sightings. Report nests even if you do not see any caterpillars, because nests are a useful sign that the pest is in the area. Do not touch 'spent' nests, which can contain large numbers of the irritating hairs.

Contact us by phone on 0300 200 7847 or by emailing to planthealth@daera-ni.gov.uk.

The Forestry Commission, England delivered a presentation on OPM at Richmond Park in London. This presentation can be accessed by clicking the link below:

Plant Health Inspection Branch issue OPM related traders notices on occasion and these can be accessed by clicking the link below:

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