Greenkeepers and groundsmen are among groups most at risk from exposure to oak processionary moth caterpillars. They are also well placed to help efforts to control the pest. Here, Andrew Hoppit, OPM project manager for the Forestry Commission, explains the issue, how you can protect yourself and your clients, and how you can help the control effort
Oak processionary moth (OPM) is a pest species which was accidentally introduced to Great Britain from continental Europe about 2005. It has become established in oak trees in parts of West and South-West London, the Croydon/Bromley area of South London and in the Pangbourne area of West Berkshire. Affected sites include high-profile properties such as the Royal Botanic Gardens Kew and Richmond Park.
OPM larvae, or caterpillars, are a threat to tree, human and animal health. They feed on oak leaves and, if allowed to reach plague numbers, can strip entire oak trees bare, leaving them vulnerable to other threats.
The larvae's tiny hairs contain an irritating protein called thaumetopoein, giving rise to the insect's scientific name, Thaumetopoea processionae. These are barbed and can be blown off by the wind, ejected as a defence mechanism, and left in nests and shed skins.
Contact with them can cause itchy skin rashes and eye and throat irritations, including occasional breathing difficulties, in people and animals. The symptoms vary in severity, and repeated contact can lead to sensitisation (i.e. they can increase in severity each time).
Clearly, then, groundsmen and greenkeepers with oak trees on or close to the properties they manage need to be aware of:
- the risks to themselves and users of their grounds
- the measures they can take to protect themselves and grounds users
- the guidance for dealing with infested or potentially infested trees
In addition, the fact that they work close to trees means they are ideally placed to act as eyes and ears in support of our control programme.
Thankfully, although we know of individuals who have been affected enough to seek medical advice, no serious OPM health issues or events have been reported in the UK. We believe this is because we and our partners, especially local authorities and major landowners, have learned from experience in continental Europe, and been proactive and effective in our efforts to keep the population down and the outbreak areas contained as much as possible.
The main danger period is from April, when the larvae start emerging from eggs laid high in oak trees by adult moths the previous year, through to the end of summer. However, care should be exercised throughout the year, because 'spent' nests and shed skins can remain on trees - or on the ground if they have fallen out - and contain thousands of the hairs. Hairs, and nest and web material, can stick to bark and remain a threat there for some time.
Protecting trees, people and animals from OPM exposure starts with vigilance and risk assessment. Professional groundstaff will naturally want to protect themselves and their oak trees, but they are also in the front line for discharging property owners' duty of care to the users of their grounds. These can range from sports participants, spectators, students, teachers, hospital staff and patients to dog walkers, joggers and child minders and their infant charges.
The best protection is:
- constant vigilance for signs of the pest, especially in spring and summer
- rapid action to remove it if it is identified
We therefore recommend that groundstaff in the known affected areas, and within two kilometres of them, incorporate into their routines a regular programme of inspecting oak trees for signs of OPM.
In any case, anyone involved with pruning or removing oak trees in OPM-affected areas must be familiar with OPM at its various stages, and undertake a visual assessment to determine whether OPM is present. Key evidence of OPM can include:
- egg masses - although these are smaller than a fingernail and very difficult to see
- larvae (caterpillars) - typically clustering together or moving about in nose-to-tail processions
- silken webbing nests and trails on oak trunks and branches. These are white when fresh, but become discoloured with time. They can fall out of trees, so look for them on the ground as well
OPM evidence can be difficult or impossible to see early in the season, especially from the ground and in poor light, when the only presence might be egg masses or tiny, juvenile larvae high in the tree. So inspect oak trees closely - binoculars can be useful. And, if you find it, report it. (See 'Legal requirements' below.)
And don't jump to the conclusion that trees are free of the pest just because you can't see anything. Your mantra within or near the known affected areas, especially if you are planning to work on trees, must be: "don't take the chance - wear PPE" (personal protective equipment).
We advise anyone working on trees in the known infested areas, or within two kilometres of them, to wear PPE as a precaution. Key among PPE items are:
- filtering face masks and goggles
- impermeable protective suits
- chemical-resistant gloves and rubber boots
In addition, don't forget that:
- although the tree you are working on might not be an oak, its near neighbours might be - and, if they are infested, you can still be exposed to wind-blown hairs
- dogs are inquisitive, and their suffering can be severe if they get OPM hairs in their mouths and noses. So, if your dog accompanies you to work, don't expose it to the risk
With support from the Arboricultural Association and the London Tree Officers' Association, we have published good-practice guidance for arborists, tree surgeons and pest control operators at www.forestry.gov.uk/opm. We recommend that groundsmen and greenkeepers who work in or close to the affected areas familiarise themselves with this guidance, especially if they include tree care among their responsibilities. It contains summaries of:
- the risk assessment precautions to take before beginning work on trees which contain or might contain OPM
- the minimum recommended specification for PPE
- the key regulatory requirements for working with OPM-infested trees, covering topics such as notifications of infestations and handling oak material
OPM is a legally notifiable tree pest, so suspected sightings must be reported to us or to your Local Council. No work may begin on a suspected infested tree until we have inspected it.
If you have already started work on the tree before seeing OPM evidence, you must stop work and report it. However, making a dangerous tree safe takes priority, but you must still report the suspected infestation as soon as possible.
Clearly marking the affected tree can help us to ensure the correct one is treated. If infestation is confirmed, we will issue a statutory Plant Health Notice to the owner requiring its removal, and you must follow the Good Practice Guidance after that has been done.
Sightings are best reported via:
- our Tree Alert app or on-line reporting form - see www.forestry.gov.uk/treealert
- the Local Council. A directory of councils' reporting contacts is available under 'Reporting suspected cases' at www.forestry.gov.uk/opm;
We particularly welcome sighting reports with clear photographs to aid identification, and precise locations - OS grid references are ideal. Trees should be marked to ensure we inspect the correct ones
We monitor male moths with a network of pheromone traps, which can provide clues to where we might expect to find OPM the following year.
There are restrictions on the handling of oak material, such as felled trees and prunings, in the affected areas. These are intended to prevent accidental spread of OPM, for example, by larvae or adult moths which might be present in felled trees or prunings. The restrictions are set out in detail in the Good Practice Guidance, but key points are:
- no work may be carried out for twelve months on a tree which has had OPM presence confirmed and treated, to allow time to ensure that it is free of OPM. If the work cannot be delayed this long, e.g. for safety reasons, the arisings must be chipped before removal
- oak material in the affected areas should be kept on site, if possible, until movement no longer risks spreading the pest. If this is not possible, the arisings must not be taken outside the affected areas without first consulting the Forestry Commission's Plant Health Service. Email email@example.com or telephone 0300 067 5155.
If you bring in tree surgeons, make sure they are reputable, such as members of the Arboricultural Association, and check that they are familiar with these requirements. The association has a directory of members at www.trees.org.uk/find-a-professional/Directory-of-Tree-Surgeons.
Control policy and methodology
The Forestry Commission leads the control programme because OPM is a notifiable tree health hazard. We work with Public Health England and other health and local authorities, the Greater London Authority (GLA) and major landowners to devise, implement and develop the control programmes.
In London, we are advised by the OPM Advisory Group, which is chaired by the City of London Corporation and comprises representatives of many of the above organisations. In Pangbourne, we work most closely with West Berkshire Council.
We aim to eradicate the Pangbourne and Bromley/Croydon outbreaks, and to contain and limit the spread of the larger West and South-West London outbreak. We are particularly anxious to prevent OPM becoming established in woodland, where it would be much more difficult to identify and control than in parks, gardens, hedgerows, streets and farmland.
The control programme comprises five key phases:
- winter surveying and plotting of egg masses and old nests
- spring surveying and plotting of egg masses and emerging larvae
- late spring and summer spraying of known infested trees and all oak trees within 50 metres of known infested trees
- late summer surveying and manual removal, usually by vacuum equipment, of nests, whose presence indicates the possibility of missed infestations
- late summer and early autumn pheromone trapping for adult male moths
Most control operations, such as spraying and nest removal, are carried out by pest control professionals with appropriate training and equipment. In addition, some owners of large landholdings have obtained their own equipment and trained their own staff in these tasks.
Groundcare professionals working within or near to the affected areas are well placed to provide an invaluable supplement to our formal surveys. Our surveyors cannot inspect all the many potential host trees, so we rely on, and are very grateful for, the vigilance and reporting of others, especially groundcare and treecare professionals, to build as full a picture as possible of the pest's distribution.
Further information, including photographs to aid identification and a report of the 2013 control programme, is available at www.forestry.gov.uk/opm. Local authority groundstaff can also consult their tree officer colleagues, whose professional association, the London Tree Officers Association, is closely involved in the control programme. (www.ltoa.org.uk)
OPM - KEY FACTS
• Native range - Southern Europe
• Northernmost presence - northern Germany and Netherlands
• First introduction to UK - probably 2005, to West London, probably as eggs previously laid on semi-mature oak trees imported from continental Europe
• Habitat - almost exclusively oak trees. Will attack other broadleaved species if preferred oak-leaf diet is lacking
• Distinctive feature 1- Larvae move about in nose-to-tail processions, hence the name
• Distinctive feature 2 - white, silken webbing nests and trails on oak trunks and branches. Larvae rest in these when not feeding, and during pupation into adult moths in early summer
• Not to be confused with - pine processionary moth (no evidence of establishment in UK)
Images courtesy of Forestry Commision, Richard Gill/SheffieldCity Council, Henry Kuppen, Martin Townsend, Mark Townsend