Identification, risks and removal. Everything landowners, property managers and homeowners across Hertfordshire, Bedfordshire and Buckinghamshire need to know about OPM, written by the team that removes it.
Oak processionary moth (Thaumetopoea processionea, usually shortened to OPM) is a moth whose caterpillars live and feed almost exclusively on oak trees. It arrived in London in the mid-2000s on imported oaks and has been spreading outwards ever since. The caterpillars get their name from their habit of moving nose-to-tail in long processions along branches and trunks.
Two things make OPM a genuine problem rather than just another garden insect. First, the older caterpillars carry thousands of tiny hairs containing an irritating protein called thaumetopoein. These hairs break off easily and can be carried on the wind, so you do not need to touch a caterpillar to be affected. Second, a heavy infestation can strip an oak of much of its leaf cover, leaving the tree weakened and more vulnerable to drought and other pests.
OPM is only found on oaks. The caterpillars will occasionally spill onto neighbouring trees such as hornbeam, hazel, beech, birch or sweet chestnut when they have stripped an oak bare, but they cannot normally complete their development on anything other than oak. If the tree in question is not an oak, it is almost certainly not OPM.
OPM produces one generation a year, and every stage of that cycle affects what you should do about it, and when.
Eggs hatch from around late March into April. The young caterpillars feed high in the canopy and mostly go unnoticed.
The caterpillars grow, develop their irritating hairs, form processions and build the distinctive white silken nests. This is the highest-risk period for people and pets.
The caterpillars retreat into the nests to pupate. Adult moths emerge a few weeks later, fly, mate and lay their eggs high up in oak trees.
OPM overwinters as eggs on oak twigs. The old nests stay on the tree, and the hairs inside them stay hazardous.
The bit most people miss: an empty nest is not a safe nest. The hairs keep their irritating properties long after the caterpillars have gone, so old nests from previous seasons still need treating with the same respect as active ones.
Every summer we get calls about “OPM” that turns out to be a harmless native species. We would much rather tell you it is nothing than sell you a removal you do not need, so here are the usual suspects.
Small ermine moths cover whole hedges and shrubs in sheets of silk webbing, which looks alarming but is completely harmless. The caterpillars are cream with black spots, hairless, and never found on oak.
Brown-tail moth builds tent-shaped webs at the tips of branches on blackthorn, hawthorn and bramble, not rounded nests on oak trunks. Its hairs can cause an itchy rash, so still keep your distance, but it is a different pest with different treatment.
Lackey moth caterpillars are brightly striped in orange, white and blue, nothing like OPM’s grey. They spin tent webs between twigs on fruit trees, hedgerows and occasionally oak.
Buff-tip moth caterpillars are yellow and black, appear from mid to late summer, and build no nest at all.
Vapourer moth caterpillars are unmistakable close up, with four pale tufts like shaving brushes along the back. They live alone, with no processions and no webbing.
Not sure what you are looking at? Take a photo from a safe distance and send it to us. If it is nothing, we will tell you it is nothing.
The hairs on older OPM caterpillars contain thaumetopoein, an irritating protein. Contact with the hairs can cause itching skin rashes, eye irritation, sore throats and, in some cases, breathing difficulties. Because the hairs are shed into nests and carried on the wind, you do not need to touch a caterpillar to react, being near an infested tree can be enough.
The risk is highest in May and June, when the caterpillars are active and feeding, but the hairs stay active in nests all year round, including old, empty ones.
Dogs investigating nests or fallen caterpillars with their noses come off worst, and grazing animals under infested oaks are also at risk. If your dog has been nosing around an oak and starts pawing at its mouth, drooling or developing swelling, call your vet.
We covered the 2026 season’s outbreak in more detail in our post, Oak Processionary Moth Warning: What Hertfordshire Homeowners Need to Know.
England is divided into three OPM management areas: an established area in the South East where the pest is now resident, a buffer zone around it with an annual Forestry Commission programme of surveillance and control, and an area officially free from the pest covering the rest of the country.
The rules differ depending on where your trees stand. In the established area, managing OPM on your land is your responsibility, and larger landholdings are expected to have a management plan. In the buffer zone and pest-free area, sightings must be reported, and the Forestry Commission can issue a Statutory Plant Health Notice requiring action. There are also restrictions on moving larger oak trees between zones.
The zone boundaries are redrawn periodically as the moth spreads, so rather than repeat a map that may date, check your postcode against the current one in the government’s managing OPM in England guidance.
Suspected OPM should be reported to the Forestry Commission through TreeAlert. It is free, it helps track the spread, and it is expected of you in the buffer and pest-free zones. We report sightings from our own surveys as a matter of course.
For estates, schools, golf courses, block managers and councils, OPM is an occupier’s liability and duty-of-care issue as much as a tree health one. If people work under or near infested oaks, government guidance expects risk assessments, suitable protective equipment and health surveillance for those workers. Knowing about an infested oak on a boundary path and doing nothing about it is not a comfortable position to defend.
Professional OPM control comes down to two tools, used at the right time of year.
Insecticide treatment works on young caterpillars, typically from around mid-April into May. Timing matters: spray too early and the eggs have not hatched, too late and the caterpillars are large, haired and sheltering in nests. Spraying after the moths have flown achieves nothing at all.
Nests are removed by trained operators wearing sealed protective clothing and respiratory protection, using controlled removal methods that stop the hairs from spreading. The material is sealed and disposed of properly, not dropped in a wheelie bin. The prime window is late summer, once the caterpillars have pupated inside the nests, because removing a nest then takes the pupae with it before they can emerge as moths. Old nests can be removed through autumn and winter.
Government guidance is blunt on this point: OPM should only be handled by appropriately qualified and equipped professional operators, and it points landowners to Arboricultural Association contractors to find one. Evoke is an Arb Association Approved Contractor.
Old nests are easier to spot on bare branches. A good time for winter surveys and removing last season’s nests.
Eggs hatch. The window for effective spraying opens around mid-April. Book surveys now if you manage multiple oaks.
Peak risk. Processions, fresh nests and airborne hairs. Keep people and pets clear of infested oaks and report sightings.
Caterpillars pupate inside nests. The prime removal window opens, taking out nests, hairs and pupae in one job.
Moths have flown and laid eggs. Nest removal continues. Plan next season’s management for larger sites.
Eggs overwinter on twigs. Old nests remain hazardous and can still be removed safely.
We are a Welwyn Garden City tree surgery firm, and this year OPM stopped some of our own jobs mid-way. Rather than keep subcontracting the problem out, we trained and equipped our own team, so the people who find the nests are the same people who remove them.
The caterpillars’ hairs contain an irritating protein and can cause skin rashes, eye irritation, sore throats and breathing difficulties. Most reactions are unpleasant rather than serious, but people with asthma or allergies can be affected more severely. The caterpillars and nests should never be touched.
No. Disturbing a nest releases thousands of irritating hairs into the air, and household waste routes cannot deal with the material safely. Government guidance says removal should only be done by trained, equipped professional operators.
Late summer, after the caterpillars have pupated inside the nests, because removal then also takes out the pupae before they emerge as moths. Old nests can be removed through autumn and winter, and they stay hazardous until they are removed.
It depends where you are. In the established area of the South East, managing OPM on your land is your responsibility. In the buffer zone and beyond, you must report sightings and the Forestry Commission can require action through a Statutory Plant Health Notice. Landowners also owe a general duty of care to people on or near their land, which is hard to square with ignoring a known infestation.
Rarely on its own. Heavy infestations strip leaves and weaken the tree, leaving it more vulnerable to drought and other pests and diseases. The bigger immediate issue is the risk the hairs pose to people and animals near the tree.
It depends on the number of nests, their height and the access, which is why we survey first. The inspection is free, you get a firm price before any work happens, and if what you have is not OPM, we will say so and leave you in peace.
Keep people and pets away from the tree, take a photo from a safe distance, report it through TreeAlert and get a professional to confirm it. We will inspect for free anywhere in Hertfordshire, Bedfordshire, Buckinghamshire or north of the Thames.
The tree owner is responsible for managing the tree, but hairs do not respect fence lines. The practical route is a polite conversation backed by good information, and a report through TreeAlert. We are happy to inspect and give you something factual to share with them.
We will confirm whether it is OPM, explain your options honestly and give you a clear price if work is needed. We respond within one working day.
