Protecting Our Trees. Protecting Our Heritage.
A dedicated UK resource for anyone who has found something unusual on an oak tree — and for the organisations responsible for keeping people safe beneath them. Calm guidance, official reporting routes and professional management support.
Think you've found OPM?
Most people arrive here having seen something unusual on an oak tree and wanting to know whether it matters. Work through these steps calmly — nothing here needs to be done in a hurry, and nothing here asks you to get close to the tree.
We aim to respond within 48 hours.
Do not touch, disturb or attempt to remove suspected caterpillars or nests.
If it is safe to do so, take clear photographs from a distance.
Do not climb trees or put yourself at risk simply to obtain a photograph.
If you are unsure what you have found, send us your photos.
Suspected Oak Processionary Moth sightings should be reported through the official TreeAlert reporting system.
Some sites need more than guidance — particularly where several oaks or busy public areas are involved.
The two routes
We can help you work out what you are looking at. TreeAlert is where suspected sightings are formally recorded. Most people need both.
Not sure what you've found?
Send clear photographs of the tree, the caterpillars or the suspected nest and our team will give you an initial visual opinion to help you understand your next steps.
Photo identification is preliminary and may not always be conclusive. A professional site inspection or official confirmation may be required.
Think it may be OPM?
Suspected Oak Processionary Moth sightings should be reported through TreeAlert, the official tree pest and disease reporting service. This is an external official service and opens in a new tab.
Sending us photographs does not replace official TreeAlert reporting.
Understanding OPM
A non-native moth, established in parts of the UK, whose caterpillars are both a tree health issue and a public health concern.
Oak Processionary Moth (Thaumetopoea processionea) was accidentally introduced to the UK and is now established in parts of London and surrounding counties, with occasional findings further afield. Its caterpillars feed on oak foliage and move in the distinctive nose-to-tail processions that give the species its name.
The concern is not simply defoliation. Each caterpillar carries thousands of microscopic hairs which can cause itching rashes, eye irritation and respiratory discomfort. Those hairs remain in nests and shed skins long after the caterpillars themselves have moved on, which is why old nests are still treated as a hazard.
For most people, the practical questions are simple: is this OPM, does it matter where it is, and who needs to know? That is what this site is built to answer.
Identification
Six things to look for. Species, host tree and behaviour together are far more reliable than appearance alone — and several harmless native caterpillars are regularly mistaken for OPM.
Grey-brown bodies covered in very long white hairs, with a darker head and a distinct dark line along the back in later stages. They are almost always found in groups rather than alone, and they are found on oak.
The behaviour the moth is named for: caterpillars move nose-to-tail in single file, sometimes in wide arrowhead formations, up and down the trunk between feeding areas and the nest.
Silken white webbing tents on the trunk or branches, tightly matted against the bark rather than loose and airy. Older nests discolour to a dirty brown and can persist long after the caterpillars have gone.
Feeding produces ragged, skeletonised leaves and, in heavier years, noticeable defoliation of parts of the canopy. Healthy mature oaks usually recover, but repeated defoliation adds stress.
Eggs are laid in neat plaques on twigs high in the canopy and are covered with scales that camouflage them against the bark. They overwinter there and hatch as the oak comes into leaf.
Several harmless native caterpillars are regularly mistaken for OPM, including tussock moths, brown-tail moth larvae and the webbing of spindle ermine. Species, tree and behaviour together tell you more than appearance alone.
Interactive guidance
Choose the type of site where you have seen suspected OPM and we will set out the practical steps in the order that makes sense for that setting.
Private Garden
Seasonal behaviour
Emergence follows the oak, not the calendar. Understanding the stages explains why inspection timing matters and why old nests still count.
Late summer to early spring
Eggs sit in camouflaged plaques on twigs high in the canopy through autumn and winter. They are extremely difficult to see from the ground.
What this means for inspection
Winter is when climbing inspection or close canopy assessment can locate egg plaques, and when old nests are easiest to spot on bare branches.
Timings are indicative for the UK and vary by season, region and local weather. Always base inspection on conditions on site.
Who we help
A single garden oak and an oak-lined golf course present the same species and entirely different management problems.
Management solutions
Effective management is a programme rather than a single visit: know your trees, inspect them at the right time, and act where exposure justifies it.
Professional visual inspection of oak trees from ground level, with binoculars where canopy height requires it, recording activity, nests, feeding damage and position relative to people.
Identifying and recording every oak across a larger property so inspection is repeatable, nothing is missed between seasons, and findings can be compared year on year.
Classifying oak trees by observed activity, public exposure, tree location, how the site is actually used and the resulting potential risk — so effort goes where it matters.
Repeat inspection timed to the relevant stages of the OPM life cycle rather than to fixed calendar dates, because emergence follows the season locally.
Pheromone traps can indicate adult male flight activity on a site and help inform monitoring decisions. They are a monitoring tool within a wider strategy — they do not control or eliminate OPM on their own.
Assessing where intervention or professional removal is appropriate, based on exposure and access rather than the presence of a nest alone. Removal is specialist work with respiratory and skin risks.
Temporary, proportionate access control around higher-risk trees — practical barriers, signage and route changes that people will actually follow.
Structured plans for organisations managing multiple oak trees, covering inspection timing, thresholds for action, documentation and annual review.
The framework
The structure we use with organisations responsible for multiple oak trees and public access.
Establish what you actually have: every oak, its location, approximate size and the ground use beneath it.
A first full inspection across the inventory, recording activity, nests and feeding damage tree by tree.
Rank trees by exposure and observed activity so attention concentrates where people and trees meet.
Repeat inspection at the life-cycle stages that matter, with findings recorded against the same tree records.
Act where exposure justifies it — exclusion, nest management or professional removal — not everywhere by default.
Keep a defensible record of what was inspected, what was found, what was decided and who was informed.
Review the season, adjust priority trees and inspection timing, and carry the record forward.
Professional support
Professional support is available for sites requiring inspection, monitoring, risk management or safe nest removal. Most enquiries begin with a conversation about the site rather than a quotation.
Supported by
OPM Advice & Management UK is supported by Hertfordshire Pest Control, who provide the professional inspection, monitoring and nest management capability behind the advice on this site.
OPM Advice & Management UK is an independent information resource and is not part of, affiliated with or endorsed by Defra, the Forestry Commission, Forest Research or any local authority. Official organisations and resources are referenced to help visitors access authoritative information. Suspected OPM sightings should be reported through the appropriate official reporting channels.