Oak processionary moth - questions and answers

Can OPM be fatal to humans?

We are not aware of any deaths directly attributable to this species.

How far could it spread in Great Britain?

Although scientists believe it is theoretically possible that OPM could survive and breed in much of England and Wales, it remains confined to a small number of sites in the South and West of London and one in Berkshire. We are currently working very hard to ensure it does not spread, and to support that, the Government announced a £1.5 million project on 9 May 2013 to treat oak trees in affected areas.

How badly can it damage trees?

Like all defoliators, it feeds on the leaves of oak trees. High populations can cause severe loss of foliage, but it is not usually fatal to affected trees, and they will produce leaves the following spring and recover. However, the weakening of the trees might combine with other factors such as other pests, disease and drought to cause a decline in trees’ overall health. Indications from attacks in Kew Gardens are that it will attack many species of oak, including those grown for timber and those for amenity purposes.

Does it or could it affect any other species of tree?

It has been recorded in continental Europe on hornbeam, hazel, beech, sweet chestnut and birch, although mainly where they have been growing next to severely defoliated oak trees where their preferred food supply of oak leaves has become limited. It is possible that it might attack a new plant or tree species. This is one of the reasons why it is important to monitor the situation and to collaborate with researchers in other countries where it occurs.

Who can do the work?

Usually pest management experts who are fully compliant with the relevant pesticide and health and safety regulations, and trained and equipped to do the work safely and effectively and in a way that safeguards human health, the trees and the local environment.

Can I treat the nests or caterpillars in trees in my own garden or property?

We strongly advise against this because of the health dangers posed by the caterpillar hairs, thousands of which can be in the nests, and because, to be as effective as possible, the job must be done at just the right stage in the moth's lifecycle by someone trained to do it correctly.

Does it have any natural predators?

Oak processionary moth does have some natural enemies in its native range. These include ‘generalist’ predators such as some species of birds, beetles, small mammals and parasites. We can expect some of our ‘generalist’ predators and parasitoids to exploit populations of the moth here, but these might not have an overall effect on its population dynamics. We will investigate the potential use of natural enemies as part of a longer-term management strategy if it does not prove possible to eradicate or contain it effectively by direct action. However, any such proposal needs careful study and handling to ensure that introducing such an organism to Great Britain would not have adverse impacts.

What measures were in place to stop it coming into Britain?

There were, and continue to be, general controls in place which require that all plants imported for planting be free of harmful pests. There were no specific import controls against oak processionary moth, but the Plant Health (Forestry) Order 2005 has since been amended to impose specific requirements on oak trees being imported to ensure that they are free of the moth. This supplements the general controls.

Why did our import control measures not keep it out?

The international trade in live plants is enormous. Dryer, milder springs enable the caterpillars to survive as far north as Britain so there was always a chance that some might get here by this route. Since the eggs are difficult to spot, it is not surprising that some could arrive without been seen, especially because they were not specifically prohibited and, therefore, would not have been subject to inspection for this pest. Unfortunately, once any eggs hatch there is a ready food source for the young caterpillars on the very trees on which they were imported.
Although much less likely, it is also possible that a gravid (egg-bearing) female moth could enter the country in a plane, train, ship or road vehicle, or be blown across the English Channel, and lay her eggs on a tree in Britain. Our import controls are kept under constant review to ensure they are as effective as possible.

Are our import control measures and their enforcement working?

Under various international agreements, and the EU Plant Health Directive, import controls generally only specifically target known pests that pose a significant threat to the health of plants and trees. There are requirements in place designed to ensure that all planting stock, from any source, is inspected before being moved in trade to ensure it is free of both listed pests and others which pose a threat. However, given the sheer volume of imported plants and trees, there is always a risk that some pests will be inadvertently overlooked during inspections.

What criteria are used to formulate import controls?

Import controls are generally formulated following a process called Pest Risk Analysis, which looks at the pest, its host plant, its country or countries of origin, and its potential for causing significant damage if introduced. If it meets the criteria it is listed as a 'quarantine' pest, and import controls are formulated accordingly. Our Forest Research agency has prepared a Pest Risk Analysis for oak processionary moth, and the outcome of the UK’s request to list it as a quarantine pest is awaited from the EU.

What measures are available to stop further accidental introductions?

A pest may be declared a quarantine pest , that is, a pest with potential economic importance to an area, and which is either absent or is locally present and being officially controlled. With this status in place, the UK authorities can then impose restrictions on imports of host material, for example by insisting they be inspected before export and certified free of the pest before being shipped. We can also insist they come from places overseas that have been declared free of the pest. We may insist that certain measures be taken with imported materials. We may also ban imports of plants from certain high-risk parts of the world, and we can impose restrictions on movements of plants within Great Britain. Most of these measures have been taken for OPM.

What measures are in place to stop accidental spread from the outbreak areas?

The controls on the sale of oak trees grown in the outbreak areas are the same as those which apply to imports: oak plants must not be more than 2 metres high, and must have been inspected and found free from all life stages of the moth. The moth is less likely to colonise or lay eggs on trees smaller than this, and visual inspection is much more practicable on small trees than it is on larger, semi-mature trees such as those used in landscaping work. In addition, it is not permitted to move oak material arising from pruning or felling of oak trees in the affected areas without consulting the Forestry Commission. Please contact our Plant Health Service, 0131 314 6414, for details of requirements.

What action was taken when the pest was first found?

The pest was first found in South-West London in summer 2006, and immediate action was taken to survey the area and destroy the nests that were found. The next opportunity to take action was during spring 2007, when we commissioned surveys to find out whether the species had successfully over-wintered in the area. The finding of newly hatched caterpillars confirmed the capacity of the moth’s eggs to over-winter here, and we immediately commissioned treatment of the caterpillars, followed by a nest destruction programme in the pupal stage that summer.

Following our Pest Contingency Plan, we also convened an Outbreak Management Team (OMT) for West and South-West London in June 2007, with the objective of eradicating the outbreak. Unfortunately eradication from West and South-West London has not proved possible, and since 2011 the objective has been to prevent or slow its spread as much as possible, and keep its population and impact as low as possible. The Outbreak Management Team is now an Outbreak Liaison Group of representatives of the various authorities and managers of large land-holdings involved. We also quickly formed an Outbreak Management Team for the Bromley outbreak discovered in 2012, where we are aiming to eradicate the species from the area.

Eradication is still the objective in Pangbourne, where the scale of the outbreak is much smaller, and where our surveyors and West Berkshire Council had considerable success in locating and treating infested trees in 2011 and 2012.

Why was it decided to no longer eradicate OPM from West and South-West London?

On-the-ground experience and scientific advice after several years of tackling the outbreak led us to the conclusion that it would not be possible to eradicate the pest from this area. West and South-west London has hundreds of thousands of oak trees, many of them difficult to access, and this, combined with the brief window of opportunity to tackle the pest during late spring and early summer with finite resources, led us to conclude that eradication was no longer practicable. Ministers have accepted this advice, and we and our partners now aim to keep the population as low as possible and contain its spread as much as possible, thereby minimising the risk to public health and the area’s much-loved oak trees.

Should we avoid planting oak as a precaution?

No, there is no reason why oak trees should not continue to be planted, and stocks should be readily available from nurseries and garden centres. However, we do suggest that anyone planning large-scale plantings in the London area, especially where there is good public access, should be aware of the possibility that the moth could spread and might pose a risk to human health.

Last updated: 09/24/2014
Oak processionary moth (Thaumetopoea processionea) larva