Pine processionary moth - its progress and the threat

Pine processionary moth is heading north from the Mediterranean and experts warn it could cross the Channel, Hannah Jordan reports.

Adult pine processionary moth - image: DD Cadahia
Adult pine processionary moth - image: DD Cadahia

This week, HW reveals a new and potentially devastating pest closing in on our shores. Pine processionary moth (PPM) - Thaumetopoea pityocampa - a native of the southern Mediterranean region, has forged north and has now established colonies in pine forests north of Paris, in Brittany and around Strasbourg, where it was discovered among containerised pine trees in transit.

Like its relation oak processionary moth (OPM), PPM damages and weakens trees by feeding on their foliage. But unlike OPM it can be a direct cause of tree death and establishes in pines instead of oaks. Its hosts are predominantly Pinus pinea and Pinus halapensis, and it is the biggest cause of defoliation in pine trees - particularly young plantation and ornamental stock - in Italy.

Equally as concerning as its impact on the landscape are the tiny hairs of the PPM caterpillar that, like those of OPM, are a serious public health hazard. The hairs contain thaumetopoein, a toxin that if touched or ingested by humans or animals can cause extreme skin irritation, respiratory problems and conjunctivitis. Anaphylaxis and even one case of blindness have also been recorded.

Pic credits: 1, 2, 5 & 6: DD Cadahia, Subdireccion General de Sanidad Vegetal,; 3: John H Ghent, USDA Forest Service,; 4: François-Xavier Saintonge, Forest Health Department,

Funding diverted to exotic pest research

News of the pest's encroachment comes just weeks after the Government announced it was diverting £7m from Defra's science budget into research on combating exotic plant pests.

The funding, which will support research projects over the next three years, underpinned the launch of the Tree Health & Plant Biosecurity Action Plan - a cross-sector initiative run in partnership with the Forestry Commission.

It is this funding that would likely support an investigation into the level of threat posed by PPM. After watching the pest's expansion across Europe, the commission is considering ordering a formal pest risk analysis to gather evidence that would support a request for tighter EU barriers against PPM crossing the channel.

Forest Research Wales branch head Hugh Evans explains that the plant health authorities can order the destruction or treatment of any plants found in the UK harbouring PPM because it is not a pest normally found on these shores. But he adds that it is precisely because it is not found here that there is no legal requirement for imported plants to be specifically inspected for PPM, meaning that it has to enter the country before we can act on it.

"This is what happened with OPM," he points out. "It wasn't listed before it arrived here because it was considered too hard for it to cross the channel, but the increasing trend of shipping plants over from Europe is making it much easier."

Evans cites milder winters as another reason behind PPM's spread into northern European. "A favourable climate and suitable woodlands of Corsican and Scots pines would make southern Britain particularly vulnerable," he warns.

But Evans suggests that while OPM is a serious health hazard, the threat posed by PPM to human health would likely be less severe because its larval (caterpillar) stages happen during the winter when fewer people are visiting woodlands, whereas OPM's larval stages occur during the spring and summer. The nests of the PPM are also much easier to spot and there are fewer pines in urban areas than oaks, he adds.

But Kew's head of arboriculture Tony Kirkham suggests that the potential arrival in the UK of PPM is even more of a worry than its already resident cousin. Once they have overwintered in their nests, he explains, instead of staying in the tree, PPM caterpillars form a procession down to the ground to find an area both soft and warm enough to burrow and pupate until late summer, when they emerge as moths. This pupal stage can remain dormant, extending the life cycle over two years.

"Coming down to the ground means a greater chance of human contact," says Kirkham. "I've said for a long time that this is the one we need to be on guard for - we definitely don't want it." Additionally, oaks will recover well from OPM, whereas PPM will kill pine trees outright and ultimately fairly quickly, he warns.

Arrival expected on European nursery stock

With Pinus pinea and Pinus halapensis increasingly fashionable in the UK because of the climate, Kirkham expects PPM to arrive as did OPM on nursery stock from Europe. "No-one can say this wouldn't be a real problem because we have planted more Pinus pinea in the UK in the past decade than we have in the past 100 years - a lot of nurseries are selling it," he says.

Swift and aggressive action will be the only way to deal with the pest should it emerge, he maintains. "We have to go into full eradication mode immediately. If it's found on young planted nursery stock, they should be destroyed straight away. We shouldn't be trying to spray and retain the tree."

Ultimately, Kirkham insists that "at risk" imported trees should be quarantined for 12 months without planting in the ground. It is a theory supported by tree scientist Dr Glynn Percival, but one that he admits would be unlikely to gain growers' support. "In a perfect world we would quarantine more and increase inspections, but we simply don't have the resources. We will probably get PPM over here. It's only a matter of time," he says.

In his role as plant physiologist at Bartlett Tree Research Laboratory, Percival has worked with Kirkham to develop control strategies over the past few years for the OPM outbreak at Kew Gardens, and he feels that the only way to control a PPM outbreak would be with speed and force.

"We have to hit it really hard at the first outbreak. The only treatment that has ever given 100 per cent results for OPM is deltamethrin, so if PPM arrives we should be spraying with it immediately," he maintains. "I'm sure that this is not a view that will be appreciated by a lot of people, but it really would be the only way to control and eradicate it."

Percival explains that a major worry about the control of PPM if it arrives and, he feels, a possible contributor to the ongoing spread of OPM is a general reluctance to use insecticides.

Last week, the Forestry Commission confirmed the discovery of OPM in the London Borough of Wandsworth, bringing the number of west London boroughs - the original outbreak area - now affected to six. Increased presence in Hammersmith and Fulham has also been recorded, with infestations in Bishops Park and Hurlingham Park, while Wimbledon Common is also now affected for the first time.

The findings come after the Government abandoned attempts to eradicate the pest in the core zone this year and instead moved to a policy of containment, where management strategies are left up to the local landowners.

An eradication strategy, where the Forestry Commission serves statutory notices to landowners to remove any nests, continues within a 10km "buffer zone" surrounding the core area. The latest commission survey also found no new outbreaks outside west London or Pangbourne near Reading, where work continues to try to achieve eradication.

Pest's spread blamed on fragmented approach

The spread of OPM is down to lack of knowledge and a generally uncoordinated approach, according to Gristwood & Toms tree expert Mark Townsend. "That's why a risk analysis for PPM is a good idea - we need to learn lessons from OPM," he maintains.

He feels that there has been a lack of joined-up action from central Government, local authorities, contractors and private landowners. "They've all been doing different things and while the Forestry Commission is doing what they can, they have had their hands tied with lack of resources."

Townsend believes that neither pest will be taken seriously until the Government approaches them as human health hazards, as they are in Holland, instead of as a forestry issue. Until that time, he hopes Defra's £7m investment and some nervous stakeholders will ramp up biosecurity measures enough to stop PPM arriving.

"We need careful sourcing, good passport systems, more rigorous inspections and clever phasing of importing. It's also a question of working with European partners to have sound biosecurity at the supplier end," he says.

"OPM has really shaken some stakeholders into working more closely on this so maybe in five years time there will be a very different way of dealing with it. It doesn't help right now, but maybe some good will come from all this in the long term."

Lifecycle of the Pine Processionary Moth
Jan Feb/Mar/Apr May/Jun Jul Aug Sep Oct/Nov/Dec
3 Overwinter 4 Spring 5 Pupae 6 Adult 1 Egg 2 Larvae 3 Overwinter
nest procession sac nest
1, 2, 5 & 6: DD Cadahia, Subdireccion General de Sanidad Vegetal,; 3: John H Ghent, USDA Forest Service,; 4:
Francois-Xavier Saintonge, Forest Health Department,

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