Pest intentions: Managing Oak Processionary Moth in the UK
Eradication is no longer possible...
The management of tree health issues is rarely straightforward and Oak Processionary Moth (known as OPM) is no exception. Present in London and the south-east since around 2005, OPM is a non-native insect pest of oak trees with impacts that straddle public health, tree health, and biodiversity.
A national OPM Management Programme (OPMMP) has been in place since 2012, seeking to minimise the spread of OPM and its impact. As we pass a decade of managing this very hairy caterpillar, let’s take a look at the latest research and contemplate the future of OPM management in the UK.
A brief guide to OPM
OPM is now established in London and the surrounding counties (see current distribution here), so those who own and manage oak trees in and around the capital city are likely to be familiar with it. However, for readers based outside this area, here is a brief introduction!
The OPM caterpillar requires oak to complete its lifecycle and is therefore almost always spotted on oak trees. The species is named after its habit of processing in line from tree to tree (see below image). As well as giving the species its name, the caterpillars are the source of public health concerns. They are covered in tiny, barbed hairs, and contact with them or their nests can give rise to skin and respiratory issues in people and animals.
You are most likely to spot OPM in the spring and summer months, when caterpillars build nests on oak trees (see below image). The nests can be highly visible, so this is a good time for OPM surveys. At this time, the caterpillars feed on oak leaves so large numbers may cause defoliation of the tree. While they are unlikely to cause the death of the tree, repeated defoliation could cause issues, particularly in combination with other pressures on oak. Given the vast ecological and cultural importance of oak in Britain, it is crucial that we monitor OPM impacts and work together to protect the health of this beloved tree species.
Despite pest efforts
Unfortunately, while the majority of the UK remains free of OPM, it is no longer considered possible to eradicate it from the country. The focus is now on containing it and minimising its impact. Some of the available techniques for dealing with outbreaks - such as spraying trees with biopesticide - carry their own risks and ecological impacts, so decision-making should be well-considered and proportionate to the site-specific risk.
If OPM is discovered in the Buffer Zone or Pest Free Area (see zone map here), the OPMMP run by the Forestry Commission will work with local partners to respond effectively. The boundaries of the zones changed in March 2023 and may continue to change in future, so do make sure you’re using the current map!
In the Established Area (Greater London, and parts of Surrey, Berkshire, Hertfordshire and Essex), local authorities are now required to follow their own risk-based approaches. In locations where there is very high likelihood of contact between OPM and people (e.g. footpaths, school grounds) it might be necessary to manually remove nests or undertake biopesticide spraying to prevent harm to human health. Conversely, if an infestation is found in a location where risk of human contact is low, then it might be reasonable to simply put in place warning signs and keep the site under observation. This risk assessment process is unpacked in detail in The Tree Council’s OPM Toolkit.
Crucially, actions taken in the Established Area and Buffer Zone are helping to protect the Pest Free Area. We can all contribute to this by keeping an eye out for OPM and reporting sightings through TreeAlert.
Local challenges
Responding to pests and diseases can be challenging for local authorities, particularly when there is a public health element. With limited budgets and high workloads, tree officers can bear the brunt of increased pressures. If OPM continues to spread, increasing numbers of councils will need strategies and budgets to manage it and protect their oak trees – if possible, it is best to start thinking about this before OPM reaches your patch! Including a section on pest and disease awareness in a wider Trees and Woodland Strategy might be a good way of instilling preparedness.
We’ve been facilitating OPM workshops with local authorities, and there is clearly an appetite for information, innovation and collaborative working, but also capacity constraints. A challenge that tree officers face is communicating the complexities of OPM response to the public in order to avoid panic and over-reaction. However, there is now a wealth of practical experience and case studies to support those new to OPM management, and in coming years we hope to enhance the OPM toolkit with new, helpful information as it emerges.
Encouragingly, even in locations where there have been OPM outbreaks, there is still enthusiasm for retaining and planting oak trees. As ever, the ‘right tree, right place, for the right reason’ principle is key.
Looking to nature for solutions
Some of the brightest minds in plant health are focussing on alternative control strategies for OPM, including nature-based solutions that may offer co-benefits for biodiversity. A partnership, including Fera Science, Forest Research, Defra, the Forestry Commission and the Joint Nature Conservation Committee (JNCC), has joined forces with landowners and managers to co-design and test innovative methods. From the use of pheromones to disrupt mating, to encouraging natural predators through habitat management, it is exciting to think that our approach to OPM management will evolve and boost the natural environment. The research team is hopeful the work will enhance the range of management options available.
At The Tree Council, we are always keen to make the latest evidence and research available to those who can use it. The future of OPM management looks hopeful, and the more we can collaborate and share experience, the better.
Jess Allan is The Tree Council’s Science & Research Projects Manager. To find out more about The Tree Council, please visit treecouncil.org.uk
Further reading
There are brilliant resources available for further reading into OPM biology and management, including the OPM Toolkit available on The Tree Council website or the OPM Resource Hub, as well as lots of information hosted by Forest Research.
Depending on location, land owners and managers have certain duties if they find OPM. If you’re after the latest policy and regulation guidance, please see Defra’s webpage, which includes information about the legislative restrictions on oak tree movements and imports.
The Netherlands has been dealing with OPM for longer than the UK. You can read about their research in a JNCC report, here.
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You mention natural predators - is there any particular predator that we ought to be encouraging?