Forestry Commission
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Oak processionary moth (OPM) (Thaumetopoea processionea) is regulated as a quarantine pest known to be present in Great Britain. As a quarantine pest, it is under official control measures and cannot be introduced, moved, multiplied or released without prior authorisation.
If you suspect you have found an OPM nest or caterpillars, you must report it immediately via TreeAlert.
If you own or manage oak trees, use this guide to identify and survey the caterpillars and nests, and report any sightings.
You should read the information on government management of OPM, and restrictions on the trade and movement of oak trees.
Updated management Zones for Oak Processionary Moth, March 2024.
Moving large oak trees within the Oak Processionary Moth (OPM) management zones.
Find out about the regulations that came into force on 24 May 2023.
If you have oak trees on your land, or you work on oak trees, read how to plan and manage risks from OPM.
Oak processionary moth (OPM) (Thaumetopoea processionea) (image credit: Henry Kuppen)
What is OPM?
The caterpillars of OPM infest oak trees. They eat the leaves, which weakens the tree and leaves it vulnerable to other threats.
The nests and caterpillars are also a hazard to human and animal health.
OPM was first identified in London in 2006 and spread to some surrounding counties in south east England. See the latest distribution map and check the OPM to see which management zone your trees are in.
The moths spread by flying from one tree to the next. Usually staying close to the tree they originated from.
How to identify OPM
Caterpillars
The caterpillars have black heads and grey bodies covered in long white hairs. See these example .
OPM caterpillars
The caterpillars are about 2mm long when they emerge in spring, and tend to remain high in the trees until they are older and larger. When they reach 1cm long they develop the irritating hairs. Theyre fully grown at 2cm long.
They usually move nose-to-tail in a procession, like their name suggests. You may be able to see them on all parts of the tree, on the trunk, branches and leaves, and occasionally on the ground.
Nests
Nests are usually found in early summer, in the trunks and branches of oak trees. A tree or branch can contain many nests. They can appear at any height, from the ground to the top of the tree.
You are unlikely to find nests on any other tree or shrub species, fences, walls or other similar structures (apart from structures supporting the tree).
They are typically dome or teardrop-shaped, and range from the size of a golf ball to a rugby ball. When fresh, the nests are white with white silken trails made by the caterpillars along the branches and leaves. But the nests soon become discoloured and brown, making them harder to spot.
Nests stay attached to the tree for many months after the adult moths have emerged.
OPM nest
When to look for OPM
Late spring and early summer are the best times to spot OPM caterpillars and their new nests in your oak trees. You can look for evidence of OPM from the ground. Check the tree from several angles. Using binoculars for this is recommended.
Spring and summer
The OPM caterpillars emerge from late March to May. Theyll move down trees as they get older and bigger and build?nests.
Look for new nests in late spring and early summer when the caterpillars are active.?Focus on the branches and trunks at all heights, and the ground for fallen nests later in the summer and into autumn.
From late June to early August, the caterpillars?retreat into the nests and form a pupa (cocoon). The pupae remain in the nests until they emerge as adult moths.?
Autumn and winter
You can find used nests at any time of year, but they are easiest to spot in winter when there are no leaves on the oak tree or on the ground.
Species commonly confused with OPM
We often receive reports of caterpillars that are not OPM.
Check the guide fordistinguishing between OPM caterpillars and other species before reporting a sighting.
How to report sightings of OPM
Sightings should be reported using Tree Alert.
You can also email opm@forestrycommission.gov.uk, but Tree Alerts are preferred.
Find out what to include in your OPM report on the OPM resource hub.
We may issue a Statutory Plant Health Notice (SPHN) asking you to take action.
Well take charge of treating infested trees in the buffer zone and the area free from the pest. You can view these areas on the .
Removing nests and caterpillars is a hazardous operation. We recommend that a professional does this work.