What should tree managers do about the national spread of OPM?

Image: Andreas März (CC BY 2.0)
Image: Andreas März (CC BY 2.0)

The alarming spread of oak processionary moth (Thaumetopoea processionea, OPM) this year to more than 60 locations across the UK, confirmed last month (July 2019) by Forest Research, means the pest with the irritant hairs has now become a national problem.

"All these outbreaks seem to have been on imported oaks planted this season," says Ian McDermott, arboricultural consultant and Municipal Tree Officers Association representative. "There’s been a lack of biosecurity and the genie is now out of the bottle — it might explode next year. Tree workers will be exposed, whether on municipal contracts, private estates or domestic gardens."

McDermott says he saw a recent outbreak being dealt with on the University of Birmingham campus, ironically during a seminar on biosecurity. The university says the one newly planted Quercus robur was found to have OPM and has been removed along with five others that it says were at risk.

"Arboriculturists are trained to deal with pests affecting trees and yes, OPM is a defoliant, but more importantly it’s harmful to humans, which is not something we’re trained for," he says. "It’s a job for ‘urban foresters’ who deal with people as much as trees."

Kew Gardens, near where the first UK outbreak was spotted as long ago as 2016, now has an annual regime in which areas of affected or susceptible trees are cordoned off in spring before being sprayed from a commercial spraying rig operated by expert contractors.

"They have the expertise and resources to treat it, but no-one else can afford to. The cost is astronomical," says McDermott. "CAVAT [Capital Asset Valuation of Amenity Trees] might value a tree at £30,000, compared to £5,000 a year to spray it, in which case managers will say 'remove the tree instead'."

Detection challenge

Even detecting the pest in mature trees is challenging, he adds. "During the first and second instars they are high up in the tree and only ‘process’ down later when the hairs are a problem. So it’s expensive even to find and tree officers don’t have the resources. Most can't even do regular tree safety inspections on the recommended cycles. It’s another log on the fire, and anything you do will take resources from elsewhere."

McDermott continues: "We have upped training with the hundreds of tree wardens we have in the West Midlands. You need eyes and ears on the ground who can report sightings either to the tree officer or online via TreeAlert."

Kent-based arboricultural consultant Dr Jon Heuch believes that though "precarious", the situation can still be kept under control. "The current system of pre-notifications means that it just might be possible for Government to track down the new plantings and eradicate the new introductions," he tells Horticulture Week.

But he points out: "The pheromone traps set up by Defra are positioned on the basis that OPM will spread from its current south-east England location, so monitoring spread from multiple new plantings will require a large new setup of pheromone traps."

However, if efforts at containment are unsuccessful, which may not be apparent until next year, "the UK's ‘protected zone’ status will be threatened, and the control of oak movements may become redundant if OPM is all over the country", he says.

"If that is the case, we may have to accept that OPM is here and we will need to learn to live with it. More care will be needed when working with oak, possibly accepting that all oak may have some residual hairs. In some cases there will be pressure to fell oak trees and there may be less willingness to plant oaks in public areas."


Arboriculture Association statement

"The rapid spread of OPM is a reminder that everyone in the arboricultural profession needs to be vigilant when working in and around oaks, and that they can no longer assume that this is just a problem for London and the South East.

"The front-line nature of an arborist’s work means they are in an ideal position to spot outbreaks early. Contract managers and site supervisors should not underestimate the risk of repeated exposure. Individuals who regularly work on infected trees can potentially become so sensitised that they may not be able to continue working where OPM is present.

"Enormous care must also be taken when sourcing oak trees. If trees are imported then they should be quarantined and only purchased from responsible suppliers, and all trees should be checked thoroughly before planting out. Arborists need to be aware of strengthened national legislation to protect oak trees against OPM through movement and import, introduced in July.

"More detailed guidance on how to apply biosecurity, whatever your role in the industry, is available in the free downloadable Application of Biosecurity in Arboriculture e-book, and the London Tree Officers Association guidance note explains that operatives must wear the appropriate level of PPE [personal protective equipment] at all times when they might be exposed."

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