Oak Processionary Moth in Dorset & Hampshire: What Property Owners Need to Know
Oak Processionary Moth (Thaumetopoea processionea) is one of the most significant tree pest threats in southern England. Its toxic caterpillar hairs pose a genuine human and animal health risk, and its expanding range across Hampshire and into Dorset means Dorset property owners with oak trees need to understand the risks, know how to identify an infestation, and know what to do — and crucially what not to do — when nests are found.
This guide is written for property owners, landowners and managers in Dorset and Hampshire who need accurate, practical information about OPM. The situation is evolving rapidly — always check the Forestry Commission's current distribution maps and guidance for the most up-to-date information on local outbreak status.
What is Oak Processionary Moth?
Oak Processionary Moth (Thaumetopoea processionea) is a moth native to central and southern Europe. The adult moth is unremarkable — it is the larval (caterpillar) stage that causes harm. The species was accidentally introduced to the UK on imported oak planting stock and was first confirmed in Richmond and Kew in 2006. Since then it has spread progressively outward from the initial outbreak zone.
OPM is a regulated pest under the Plant Health (England) Order 2015. It is primarily a pest of oak trees, particularly pedunculate oak (Quercus robur) and sessile oak (Quercus petraea), though Turkey oak (Quercus cerris) and other oak species are also attacked. The caterpillars feed on oak leaves from spring, potentially causing significant defoliation in heavy infestations.
Why the Caterpillar Hairs are a Serious Health Risk
The caterpillars' hairs — technically called urticating setae — contain a protein called thaumetopoein. These microscopic, barbed hairs are shed throughout the caterpillars' development, contaminating the bark of the host tree, the ground beneath it, and the surrounding air particularly in warm, dry conditions when they become airborne easily.
The health effects of exposure include:
- Skin: Urticarial (nettle-rash type) reaction — intense itching, red rash, hive-like welts. Can develop within minutes of contact and persist for several days
- Eyes: Severe itching, redness, watering, and in serious cases conjunctivitis or keratoconjunctivitis that requires medical treatment
- Upper respiratory tract: Sore throat, irritation, coughing, difficulty swallowing — caused by inhalation of airborne hairs
- Severe reactions: In sensitive individuals, contact can trigger anaphylaxis (rare but documented). Asthma sufferers, children and immunocompromised individuals are at heightened risk
Critical safety warning: Even empty, old nests remain hazardous — the shed hairs retain their irritant properties for up to five years. Do not touch nests, do not approach infested trees closely, do not allow children or animals near OPM-infested trees. If you or anyone shows a significant reaction after exposure, seek medical advice. If severe, call 999.
Identifying OPM Infestation on Your Dorset or Hampshire Oak Trees
The Caterpillars
OPM caterpillars are grey-brown with distinctive long white hairs and a dark stripe along the back. They are found on oak trees from April to late June, moving up the tree to feed at night and returning to rest in communal nests by day. The characteristic nose-to-tail "procession" line of caterpillars moving as a group — sometimes hundreds long — is the behaviour that gives the moth its common name and is the most distinctive identification feature.
The Nests
Nests are white to off-white, silken structures that look like flattened bags or cushions. They are typically:
- Found on the main trunk or major lower branches, usually 2–6 metres from the ground
- Variable in size from a few centimetres across in early season to 30–40cm in well-established nests
- Often positioned on the south or west-facing aspect of the trunk in exposed locations
- Present from approximately May through to autumn; old nests remain attached and hazardous for years
Oak Damage
Severe OPM defoliation repeatedly over several seasons weakens oak trees and makes them more susceptible to secondary problems including secondary insect attack and drought stress. A single season of heavy defoliation rarely kills an established oak outright, but repeated annual defoliation can significantly compromise the tree's long-term health.
What to Do If You Find a Suspected OPM Nest in Dorset
Keep people and animals away
Do not approach the nest closely. Cordon off the area beneath the tree if it is in a location where people or animals pass. Do not allow children near the tree.
Report to the Forestry Commission
Submit a report via the Forestry Commission's Tree Alert portal (treealert.forestresearch.gov.uk) with photographs. OPM is a regulated pest and all suspected outbreaks should be reported, even in areas where OPM is already known to be present. The FC will advise on the management requirements in your area.
Do not attempt removal yourself
OPM nest removal requires specialist PPE (full-face respirator, disposable overalls, eye protection) and appropriate disposal of all contaminated material. All management work should be carried out by qualified professionals. Clearcut Tree Surgery coordinates with licensed OPM management contractors across Dorset and Hampshire.
Assess the tree's wider condition
An OPM infestation is an opportunity to assess the overall health of your oak trees — defoliation stress combined with any existing disease or structural weakness represents an elevated risk. Clearcut can carry out a full arboricultural assessment alongside OPM management coordination.
OPM and Tree Work — Important Seasonal Restrictions
Any tree surgery work on or near OPM-infested oak trees — including pruning, crown lifting or felling — must account for the OPM hazard. Disturbing nests or caterpillar-contaminated bark during arboricultural operations can generate significant airborne hair release. Work on infested trees should only be planned following OPM management by specialists, and operatives must use appropriate PPE. Clearcut Tree Surgery coordinates all aspects of this safely across Dorset and Hampshire.
