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Guidance: Managing oak processionary moth in England

Forestry Commission

February 9
14:22 2024

Oak processionary moth (OPM) (Thaumetopoea processionea)

If you own or manage oak trees, you should use this guidance to identify and survey the caterpillars and nests, and report any sightings.

You will also find information on government management of OPM and restrictions on the trade and movement of oak trees.

Oak processionary moth caterpillars processing up an oak tree trunk

Oak processionary moth (OPM) (Thaumetopoea processionea) (image credit: Henry Kuppen)

What is 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 management zones map 2023 with exclusion zone (PDF, 445 KB, 1 page) 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 caterpillar images (PDF, 358 KB, 1 page).

Oak processionary moth caterpillars on a leaf

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 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.

Oak processionary moth nest on an oak tree

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 summer 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

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 Species commonly confused with oak processionary moth (OPM) (PDF, 5.17 MB, 1 page) 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. We also accept reports via email at opm@forestrycommission.gov.uk, although TreeAlerts are preferred. Please read the guidance regarding the essential information to be included in OPM reports on the OPM resource hub.

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 management zones map 2023 with exclusion zone (PDF, 445 KB, 1 page). These areas have been updated and came into force on 8 March 2023.

Removing nests and caterpillars is a hazardous operation. We therefore recommend that a professional does this work. Find an OPM management professional at the Arboricultural Association.

Risks of O

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