Oak processionary moth (OPM) caterpillars are priority pests which target trees in the Quercus genus. They were accidentally introduced into England in 2005, but thanks to strict survey and control programme, they have not become established outside of the South of England. The vast majority of reports come from the London area, where Observatree volunteers have been an essential part of efforts to limit the spread.


OPM can completely defoliate and often kill oak trees. As well as being a significant risk to tree health, OPM can also negatively impact humans. Their small hairs contain a protein that causes irritation to humans and animals. This protein, known as thaumetopein, gives rise to their scientific name “Thaumetopoea processionea”.

Key Characteristics

Typically seen in May, June and July, these caterpillars are distinctive, as much for their behaviour as for their appearance. They have long white hairs (which mask the smaller, almost invisible irritating hairs), a grey body and a dark head. Older caterpillars often have a dark central stripe with paler lines down each side. Despite their distinctive looks, they can be confused with a variety of other species, which you can check out here.

Caterpillar processions are one of the key characteristics of OPM. During late spring and summer, it is possible to see groups of these caterpillars forming arrow-headed lines with one leader at the front, and several rows following behind.

OPM are almost exclusively found on oak trees, though they may also be spotted moving across the ground between oak trees.

Sweet chestnut, hazel, beech, birch and hornbeam have been secondary targets of OPM, but only once all available oak trees have been depleted. Other broadleaf trees are poor substitutes and will not sustain the full OPM life cycle as an oak tree would.

OPM nests are made in early summer, generally on the underside of branches or on the main trunk of the tree. They are made of a white, silken webbing which becomes discoloured after a short time, making it harder to spot. Nests can appear in any shape and may range from just a few centimetres to several feet across.  

What to do if you spot OPM?

The mantra for OPM is: “Spot it, avoid it, report it.” We encourage everyone to learn how to identify these species and be mindful of relevant health precautions. Sightings should be reported on TreeAlert (use Tree Check if in Northern Ireland).

If you cannot use TreeAlert, you may report a sighting by emailing opm@forestrycommission.gov.uk or telephone to 0300 067 4442.

If reporting sightings by email or telephone, please include:

  • a precise location of the tree(s) - a 10-digit Ordnance Survey grid reference is ideal, e.g AB 12345 12345. Otherwise provide a full address, including property name and/or street or road number and the full postcode; and/or
  • precise instructions for finding the tree(s), e.g. “40 metres north-west of the entrance to (name) Park in (name) Street”;
  • a telephone number where we can reach you during the daytime to clarify any points;
  • a clear, well lit photograph with email reports if you can; and contact details of the owner or manager of the tree(s), if known.