How are OPM tree and health risks being managed?

This appears to be the year that oak processionary moth (OPM) joins Japanese knotweed and grey squirrels in being widely recognised by the public as troublesome invasive species. Experts are hoping this new-found infamy will ensure continued vigilance to help keep the problem under control and keep tree workers and the public safe.

Latest map of OPM "Core" and "Control" Zones - image: Forestry Commission
Latest map of OPM "Core" and "Control" Zones - image: Forestry Commission

This year the hazardous caterpillars of OPM (Thaumetopoea processionea) were first seen emerging in mid April — later than usual because of the cold weather.

This has attracted a flurry of media interest nationally and even internationally. The Daily Mail reported a flush of the native brown-tail moth caterpillars in Cromer, Norfolk, whose hairs are also irritants, as being OPM. This also happened at Budleigh Salterton in Devon, suggesting if nothing else that public awareness of the OPM hazard is now widespread.

Contractors began treating known affected sites within a week of the caterpillars’ emergence. There are now more than 600 such sites within a considerably widened "Control Zone", which now extends from the Bedfordshire and Cambridgeshire borders in the north through nearly to Brighton in the south, and from the Wiltshire border in the west to the Medway towns in the east.

The core, or "Infested" zone, where eradication is not being attempted, remains an area extending over a dozen boroughs in west and south-west London.

The rest of the UK is meanwhile demarcated as a "Protected Zone" into which no OPM-infested material can be moved. Previous outbreaks in Leeds, Sheffield and Pangbourne, Berkshire, appear to have been successfully eradicated, with no further infestation having been found at these sites in the past four years.

Informing the public

Craig Ruddick, arboriculture manager at the London Borough of Richmond upon Thames in the heart of the Core Zone, says keeping the public and industry informed is at least as big a task as controlling the nests themselves.

"Most of my work is on the communication side — briefings, school visits, press releases, web updates," he says. "It’s been quite effective. People are taking responsibility. We can almost expand some of the risk management to the public, who can warn each other of its dangers and report it to us."

This awareness appears to have contained the public health risk, he explains. "The majority of reports of symptoms — around 40 — came in the first year, when there was a large initial outbreak. Since then we haven’t had anything near that, as we’ve kept a lid on the population, though it may be under-reported if people don’t yet recognise it. That’s part of what we’re addressing."

While welcoming the recent coverage, "some of it has gone to extremes" in descriptions of possible symptoms, he says. But he notes that repeat exposure does appear to make symptoms more severe, putting tree workers at particular risk.

"The contractors who work on controlling it are geared up with the necessary PPE [personal protective equipment], but other contractors need to be aware too," he says. "We tell our contractors they need to include it in their risk assessments, and I wouldn’t send them out to work on oaks right now."

Future development

As to how the management of the pest might develop in future, "it’s difficult to predict", he says. "It throws up something new each year. This year has been perfect for the caterpillars’ development. Now everyone thinks a caterpillar is OPM. Much depends on what the research brings on control methods and even predators."

He adds: "For local authorities, it’s about proportionate risk management. So far that’s been effective. We need to continue to raise awareness, so people learn to deal with it as they would with, say, a wasps’ nest."

Richmond is justly famous for its many mature and veteran oaks, and Ruddick says the current concern "shouldn’t lead to people removing them, or stopping planting them". He adds: "One or two people have wanted to take them out. We have refused one such application."

So far, "we haven’t had an outbreak on private land that could put the public in danger", he says. "If we did, we would approach the owner. We did take enforcement action with a private development in Kew ward during the initial outbreak." But legally the borough’s position is unclear here, he adds.

Right now though, he says, more eyes on the lookout mean better reporting and so more effective ongoing control.

Find advice and guidance on OPM at Forest Research -

Suspected OPM outbreaks should be notified to a local plant health inspector or to Forest Research via the Tree Alert service, by email to or by calling 0300 067 4442.

Have you registered with us yet?

Register now to enjoy more articles and free email bulletins

Sign up now
Already registered?
Sign in

Read These Next

Follow us on:
  • Facebook
  • LinkedIn
  • Twitter
  • Google +
Horticulture Jobs
More Horticulture Jobs

HW Top UK Arboriculture Businesses

See our exclusive RANKING of arboriculture businesses by annual turnover. 


Build your business with the latest public sector tenders covering landscape, arboriculture, grounds care, production and kit supplies. To receive the latest tenders weekly to your inbox sign up for our Tenders Tracker bulletin here.


Find out more about the outstanding parks, gardens and arboricultural projects and teams that became our Custodian Award 2019 winners.

Jeremy Barrell On...

Jeremy Barrell

Tree consultant Jeremy Barrell reflects on the big issues in arboriculture.

Products & Kit Resources