In 2020 the plant health authorities increased surveillance in areas unaffected by oak processionary moth (OPM). In June and July 2020 OPM was found on a very small number of trees at five sites, in the counties of Sussex, Oxfordshire, Bedfordshire and Cheshire. Swift and appropriate action was taken to treat the infested trees and minimise the risk of spread. This included spraying trees in the affected areas and undertaking enhanced local surveillance.

Oak trees provide significant amenity value and contribute to the health and longevity of our woodland. However, the oak processionary moth (OPM) may become a serious threat to our enjoyment of oak woodlands and can have a detrimental impact on the vigour of the trees, making them more susceptible to other pathogens. 

The OPM was accidentally introduced from The Netherlands in 2005 into two initial sites in West London and has spread to most local authorities within the M25 and many surrounding counties. The OPM is now a notifiable pest across much of the UK. Although usually found on oak trees, it can also feed on other trees such as hornbeam if population levels are high.

So what affect does the OPM have on woodland, and why is it regarded as such a significant threat?

The larvae, or caterpillars, of the OPM feed on the leaves of oak trees. Large populations can defoliate oaks, leaving the trees weakened and vulnerable to attack from other pests and less resistant to environmental factors. Oak trees support more life forms than any other native tree, hosting hundreds of species of insects as well as supplying food to birds, badgers and deer. A major decline in our oak population would have a knock-on effect on many other native species

The larvae of the OPM can be identified by their distinctive habit of moving around nose-to-tail, which is where the name originated. In early summer, they build white, silken nests on the trunks and branches of oaks – never on the leaves –which can be hemispherical, tear-drop shaped, bag-like or even like a stretched blanket.

If you suspect that your oaks may be hosting OPM larvae, then you need to tread carefully. The OPM isn’t just a danger to our woodlands, it presents a health risk to people and animals. The tiny hairs on the larvae contain an irritant protein. This irritant travels on the breeze, causing sore throats, skin and eye irritations and even breathing difficulties. If you believe that your woodland may be suffering from OPM, then never try to deal with yourself – seek professional guidance.

The key to the control of the OPM is the quick and accurate identification of the larvae. The current management of the pest entails winter surveys to identify old nests, spring pesticide spraying and summer pheromone trapping to monitor population spread. It’s a complex process to ensure the larvae are effectively controlled in the existing affected areas and to avoid spread to new areas, before they become an extensive health problem.

Further information

If you would like further information on the spread, control and management of the oak processionary moth, contact Savills Sevenoaks on 01732 879051 or Savills Petworth on 01798 345 975