Forestry Commission
Oak processionary moth (OPM) (Thaumetopoea processionea)
Oak processionary moth (OPM) (Thaumetopoea processionea) (image credit: Henry Kuppen)
If you own or manage oak trees, you should use this guidance to identify and survey the caterpillars and nests, and report any sightings.
What is the OPM?
The caterpillars of OPM infest oak trees, eating (defoliating) the leaves, weakening the tree and leaving it vulnerable to other threats. OPM nests and caterpillars are also a hazard to human and animal health.
OPM was first identified in London in 2006 and has spread to some surrounding counties including Surrey and other parts of south east England. See the latest distribution map and check the OPM to see which management zone your trees are in.
OPM moths spread by flying from one tree to the next. Normally they stay close to the tree they were on as a caterpillar.
How to identify OPM
Caterpillars
The caterpillars have black heads and grey bodies covered in long white hairs. Please see these example .
OPM caterpillars
The caterpillars are only 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, hence their name. You may be able to see them on all parts of the tree - on the trunk, branches and leaves, and occaisonally 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, best checking from several angles. Using binoculars for this is recommended.
Spring and summer
The OPM caterpillars emerge from late March to May. Theyll move down the 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 season.
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
Look for used nests in winter as they are easier to spot when there are no leaves on the oak tree or on the ground, but you can find them at any time of year.
Species commonly confused with OPM
We often receive reports of caterpillars which are not OPM, please take time to check this guidance fordistinguishing between OPM caterpillars and those of other species before submitting any reported sighting.
How to report sightings of OPM and what happens after you file a report
Any sightings should be reported to the Forestry Commission via the Tree Alert online form. Alternatively, people can email opm@forestrycommission.gov.uk or call 0300 067 4442.
The Forestry Commission may issue you with a Statutory Plant Health Notice (SPHN) asking you to take action. This document will explain what action is needed.
In general, the Forestry Commission will take charge of treating infested trees in the buffer zone and pest free area. You can view these areas on the . These areas come into force on 1 March 2022.
Removing nests and caterpillars is a hazardous operation. We therefore recommend that a professional does this work. You can find a professional on the Arboricultural Association website. They may spray the trees with an approved bio-pesticide or insecticide.
Risks of OPM
Do not touch or get close to the caterpillars or nests.
OPM caterpillars and nests affect the health of people and animals, as they contain hairs which can cause itchy rashes, eye and throat irritations and occasionally breathing difficulties in both people and animals.
In rare cases the hairs may cause an allergic reaction. You can become sensitised by repeated exposure to the hairs, worsening your symptoms.
People or animals can come into contact with OPM hairs if they touch OPM caterpillars and their nests by brushing past an infested oak tree or are near infested trees as hairs?can be blown about in the wind.
Animals can get hairs on their paws, and hairs can also get into their mouth and nose by sniffing, licking and picking up?caterpillars or nests causing irritation. Livestock and horses can come into contact with hairs on the grass or other?plants they eat, and by grazing close to a fallen nest