Battle tactics - combatting oak processionary moth

The Dutch experience of oak processionary moth offers some guidance for the UK, says Gavin McEwan.

Methods used to control moths include spraying a pesticide that is active for 7 days - image: Klippen Boomverzorging
Methods used to control moths include spraying a pesticide that is active for 7 days - image: Klippen Boomverzorging

With the Forestry Commission switching from a policy of eradicating oak processionary moth (Thaumetopoea processionea) to one of containing it (HW, 11 March), the experience from the continent suggests that the tree care industry is facing a challenge, in both cost and hazard terms.

The moth's caterpillars release what are known as urticating hairs ("urtica" is Latin for nettle) when disturbed. Less than a tenth of a millimetre in length, the hairs have small barbs that hook onto skin and other exposed parts, causing rashes that can last weeks or even months. They can also cause a more serious anaphylactic reaction - and there has been at least one case of blindness.

Despite this, the problem is not considered a public health issue in the UK, unlike in the Netherlands, which has been colonised by the moths over the past two decades. There, Kuppen Boomverzorging (Kuppen Tree Care) specialises in controlling the pest, and it was to founder Henry Kuppen that UK tree care specialist Gristwood & Toms turned for expertise when it became apparent that similar methods would be required on an ongoing basis in this country.

The two concluded a "partnership agreement" last year. "I think it was only a matter of time before the UK got it too - it likes the climate here," Kuppen told a recent conference on the moth at Gristwood & Toms' Hertfordshire headquarters.

A complex problem

Explaining the many-faceted nature of the problem, Kuppen says: "People doing the eradication are at high risk, whenever they do it, as the hairs persist in the nests. And you react worse each time - some of my guys can't work around oaks any more.

"Then there's the general inconvenience to the public. Dogs and horses are also very sensitive to the hairs, which they sniff up from the ground. Then there is the cost of medical care and lost opportunities for recreation, study and work where areas are closed off."

The moth first appeared in the southern Netherlands in 1989, prompting a control programme in 1991, but this failed to stop its advance, which happened at a rate of around 7.5km northwards per year. "Now the Netherlands is completely covered," says Kuppen. In the UK, the problem was first detected in the London Borough of Richmond in 2006, but is still largely confined to west London.

"We have an enormous number of oaks. In some areas, they are three-quarters of the tree population - much more than in countries further south," says Kuppen. "It became clear that, short of cutting them all down, we couldn't get rid of the problem."

Dutch municipalities are responsible for control measures, which in 2007 typically cost EUR1,000-EUR10,000 per municipality, or around EUR2 (£1.70) per tree. The problem is not completely restricted to Quercus because while the moth prefers European deciduous oak species, it will also take up residence in Prunus, Fagus or Betula, he explains.

As a European pest, the moth does have natural parasites. "With enough, they will stabilise the problem, but won't eliminate it," says Kuppen, adding that a beetle, the forest caterpillar hunter (Calosoma sycophanta) "is coming back to the Netherlands" in the moth's wake, along with parasitic larvae of tachinid flies. "Bats and birds will eat the caterpillars until L3 (a stage of the pest's life cycle, see box above). After that, they throw up."

Effective control

He outlines five control methods that his company has employed with varying degrees of success. The first involves an electrostatic spray of Bt (Bacillus thuringiensis) based pesticide, the droplets being given a positive electric charge to bind them to the weakly negative-charged trees, minimising drift. The toxin remains active for seven days. "We will spray this way for 24 hours a day for four or five weeks in season," says Kuppen. "We can do between 60 and 120 trees an hour, depending on the size."

The second method of applying a blowtorch to the nests has been used by the Belgian army - several were hospitalised. "It will blow the hairs 100m away, and at 1,000 degC it's damaging to trees, particularly younger trees," explains Kuppen. "My message is to please stop using blowtorches."

To manually pick off the nests, they must first be sealed with hairspray to stop the spread of hairs, then bagged and incinerated. "It's relatively easy and low-profile, but very labour-intensive, and there is some hair spread," Kuppen adds.

A fourth method involves sucking the caterpillars into a mobile oven that incinerates them on site. "It's quick and has high capacity and the waste ash is harmless and can be disposed of normally," he says. "But if you bury the hairs, they can still be toxic five years later. Controlling the problem effectively calls for a mix of methods."

In the UK, Gristwood & Toms is still evaluating different sprays and application methods, says contracts manager Alex Austin. These have included Deltamethrin, a broad-spectrum pesticide that remains active for 12 days.

However, regional manager Mark Townsend says Natural England will not allow Deltamethrin on its land. He adds that using the Bt-based DiPel together with Dimilin Flo "means that you need less overall".

But Townsend points out: "The cost of the chemicals is low compared to the cost of applying them. We probably won't spray this year - we are trying to find ways of proactively managing the problem."

In 2009, the company also tried manual nest removal, using hairspray to prevent the hairs drifting. It has also removed nests on 12 public and 26 private trees using vacuum cleaners to suck up the hairs, Austin adds. "The vacuums were key to the safe physical removal - they reduce though don't eradicate the health risks."

Strategic monitoring

Kuppen also emphasises the importance of data gathering to guide a control strategy. "It allows us to produce an annual nationwide overview," he says. "Even so, we have municipalities that aren't recording. It's been common in the Netherlands for 20 years, but still people don't know about it."

An online GPS-based recording application,, collates data in the Netherlands on complaints and control measures, which can be logged on-site using a personal data assistant.

Kuppen has also developed a risk profile map, with each area graded from one to three, depending on whether it includes facilities such as schools. "It dictates how fast you have to act," he explains, adding that areas such as Amsterdam's Vondelpark, which draws four-million visitors a year, are regularly inspected as a precaution.

Besides visual inspection, pheromone traps placed in the upper tree canopy indicate the spread and intensity of the problem. According to Townsend: "You have to track the spread so you can target your resources, rather than just finding it on spec. Right now the recording is very piecemeal, but we are working to get an online recording system for the UK - we will pool information with other organisations."

London Tree Officers Association (LTOA) chairman Dave Lofthouse adds: "We are planning how to manage this year's outbreak. By putting all the information we have in one place, we will be better able to predict where the target areas will be - if we have the funding."

London Borough of Ealing tree section manager Ben Clutterbuck, who chairs the LTOA's oak processionary moth working group, emphasises the expense required and the need for coherent action to be taken.

"My costs were £25,000-£30,000 last year, but it could start costing a lot more," he warns. "You could be managing it, but the person next door might not be. Why invest when you may have to spend it again next year when you're recontaminated?"


The female adult oak processionary moth lays around 300 eggs in small strips or "plaques", usually in the canopy of a free-standing mature or semi-mature oak tree. Detection at this stage is near-impossible - even close up the 2-4cm long plaques are difficult to spot.

Not all the eggs will necessarily hatch in the same year. Once hatched, the caterpillars pass through six instars (stages):

- At L1 (April-May) the freshly hatched caterpillars are 3mm long, red and have no toxic hairs.

- At L2 (early-mid May) they have turned black but still lack hairs.

- At L3 (mid-late May) the caterpillars "process" down from the top of the tree and begin developing urticating hairs.

- At L4 (early June) they start to form silken nests, incorporating hairs and frass. They develop orange dots on their backs.

- At L5 (June-early July) silken trails are visible from the nest back up to canopy, where they return to feed.

- By L6 (mid June-July) they have reached their maximum length of 35mm. From mid June to September, they pupate in the nests, from which the adult moth emerges. It lives for four to five days.

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