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Forest Research home > Research > Protecting trees > Oak processionary moth - Thaumetopoea processionea

Oak Processionary Moth - frequently asked questions

Group of oak processionary moth larvae feeding on an oakBackground information

Does it do any harm to humans, and what should be done about this?

As well as seriously damaging trees, the caterpillars pose a risk to human and animal health. The caterpillars' tiny hairs contain a toxin called thaumetopoein, which can lead to itching skin lesions and, less commonly, sore throats, breathing difficulties or eye problems if they come into contact with people. This can happen if people touch the caterpillars or nests, or if the hairs are blown by the wind into contact with people. Further information on the potential health problems and symptoms can be found on the Health Protection Agency (HPA) website

The HPA advises anyone with a severe reaction, or who is uncertain what has caused their rash, to consult their GP.

The hairs can also affect animals, including dogs, cats and horses, so people are also encouraged to keep their pets and livestock away from infested trees.

Is it true that it can be fatal to humans?
We are not aware of any deaths directly attributable to this species.
How did it get here?
We don’t know for certain, but it was most likely by the importation and planting in Britain of living trees infested with over-wintering eggs laid by the moths.
Is climate change a factor in its arrival?
It is likely that the recent trend to milder winters, and in particular the reduced incidence of late spring frosts, which could kill significant numbers of young larvae (caterpillars), has been instrumental in the moth’s ability to survive and breed further north than its traditional native range. It might also have benefited from the exceptionally warm period in summer 2007.
How far could it spread in Britain?
It seems likely that it could survive and breed in much of England and Wales. Adult moths have been found as far north as southern Sweden, which lies at a similar latitude to Scotland, although there is no evidence that the moth has successfully bred there.
What damage does it do?
Like all defoliators, it feeds on the leaves of its host plant and can cause severe foliage loss when populations are high. However, it is generally not fatal to attacked trees, although the weakening of the trees might combine with other factors to cause a decline in their overall health. Trees will produce leaves the following spring and recover. Defoliation, therefore, can be dramatic. Indications from attacks in Kew Gardens are that it will attack many species of oak, both those grown for timber and those for amenity purposes.
Is it fatal to oak trees?
There are no known records of it killing trees in this country, although there is evidence from elsewhere that repeated attacks can reduce a tree's vigour and can occasionally prove fatal, particularly when combined with other damaging agents and severe droughts and environmental stresses.
Does it or could it affect any other species of tree?
It has been recorded on the Continent on hornbeam, hazel, beech, sweet chestnut and birch, although mainly where they have been growing next to severely attacked oak trees. It is possible that it might attack a new species, and 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.
What is its behaviour (e.g. population cycles etc)?
There is a single generation per year. Larvae (caterpillars) occur from April to June. They live gregariously in webs or nests spun among shoots or on the trunks and branches of the host plant, and wander from the nests to the leaves to feed. The caterpillars have the habit of following one another in long processions, hence the common name. Larvae pupate in the webs at the end of June or beginning of July, and emerge as adult moths 1-2 weeks later. The adults fly during July and August, during which time the females lay their eggs on host plants. The eggs over-winter and hatch during the following April.
What can we do about eradicating or controlling it?
If they can be found, egg masses can be destroyed before the eggs hatch the following spring. A more reliable method is to spray the larvae (caterpillars) with an approved insecticide soon after the eggs hatch in the spring, and this is where we concentrated our efforts in 2008 and 2009. The nests can also be destroyed during the brief pupal stage in the summer, reducing the number of adult moths that will emerge from the pupae.
What sort of operators do the work?
They are usually pest management experts who are fully compliant with the relevant pesticide and health & 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 ordinary householders treat the nests in their gardens?
We strongly advise against this because of the health dangers posed by the toxic caterpillar hairs 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 that might keep its numbers under control? Do these predators exist in Britain? If not, is there any prospect of their being introduced if the moth does become established here?

Oak processionary moth does have 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 scope the potential use of natural enemies as part of a longer-term management strategy if it does not prove possible to eradicate it 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 here.

What should I do if I find it?

Report possible sightings of the caterpillars or their nests/webs to your local Council (Brent, Ealing, Hounslow or Richmond Upon Thames) or Forest Research, who will take the necessary action. The contact details are:

If you find nests or caterpillars outside these areas, contact:

You must not attempt to handle the larvae caterpillars yourself, or disturb their nests.

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 all 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, and this, coupled with milder winters enabling the moth to survive as far north as Britain, means there was always a chance that some might get here through this route. Since the eggs are difficult to spot, it is not surprising that some could arrive without been seen, especially as they were not specifically prohibited and, therefore, would not have been subject to inspection for this pest. Unfortunately, once any eggs hatch they have a ready food source for the young larvae on the very trees on which they were imported.

Although much less likely, it is also possible that a viable female moth could enter the country in a plane, train, ship or road vehicle, or be blown across the 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 up to the job?
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, country of origin and 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 a decision on whether to list it as a quarantine pest will be taken following consideration at EU level.
What measures are available to the UK authorities to stop further accidental introductions?

A pest may be declared a quarantine pest, that is to say 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 GB authorities can then impose restrictions on imports of host material, for example by insisting they be inspected pre-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 ban imports of plants from certain high-risk parts of the world, and we can impose restrictions on movements of plants within Great Britain.

These sanctions will, however, require consideration at EU level, and we have made the necessary formal request and a decision is awaited.

What measures are in place to stop accidental spread from the outbreak area?
The controls on the sale of oak trees grown in the outbreak area are the same as those that apply to imports. Trees must not be more than 2 metres high, and must have been inspected and found free from all life stages of the moth.
Why 2 metres?
We now know that the moth will not colonise or lay eggs on trees under this size. Also, visual inspection is much more practicable than on larger, semi-mature trees such as those used in landscaping work.
Is the Forestry Commission recommending against planting oak as a precaution?
No, there is no reason why oaks should not continue to be planted, and stocks should be readily available from nurseries and garden centres around the country. 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.
What action was taken when the pest was first found?

The pest was first found in the summer of 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 larvae confirmed the capacity of the moth to over-winter here, and we immediately commissioned treatment of the larvae, followed by a nest destruction programme in the pupal stage that summer.

Following our Pest Contingency Plan, we also convened an Outbreak Management Team in June 2007. Our eradication programme is still in place, and the pest has been successfully contained within broadly the same area. We know that eradication will take some time to achieve, but our surveys have shown that the population remains at relatively low levels, and the number of nests found and destroyed in 2008 (506) was fewer than in 2007 (708).

Similar active measures are again in place for 2009, and we will assess the situation once sampling for adult male moths in pheromone traps is completed in August.

Further information

Enquiries about import controls and other regulations can be made to the Forestry Commission's Plant Health Service on 0131 314 6414 or by email to

Forestry Commission
17 June 2009