Forestry Commission logo
NEWS RELEASE No: 1660320 JULY 2016

Stay alert to chestnut tree pest

Pre-emergent oriental chestnut gall wasps (Dryocosmus kuriphilus) inside gall

The Forestry Commission is asking people to join its tree health surveyors in keeping an eye out for an exotic pest.

Oriental chestnut gall wasp (OCGW), a pest of sweet chestnut trees first found in the UK in 2015, has been spotted in South-East England.

The impact of OCGW on sweet chestnut in this country is low. It poses no threat to people or animals, nor does it affect horse chestnut trees. It can weaken sweet chestnut trees and make them vulnerable to other threats, such as drought or other pests and diseases. England’s chestnut production industry is small, with the great majority of the chestnuts consumed in Britain imported, so supply will be unaffected.

Forestry Commission surveyors have found the species at several localities in and close to London, as well as at Farningham Woods in Kent, which was the first site found last year.

The survey across South-East England will continue into August, supported by volunteer tree health surveyors from the Observatree initiative. Andy Hall, plant health manager for Forestry Commission England, said,

“Protecting our country from plant pests and diseases is important for our economy, environment and health, and we and our partners in government are committed to protecting our borders from them, and building the resilience of our trees and plants.

“The Government has invested more than £21 million into tree health research, has robust plans to deal with threats, and frequently reviews measures to minimise spread and impact.”

Mr Hall said experts are looking into long-term solutions to control populations of the tiny insect, which is less than 3 millimetres long, and added,

“We cannot eliminate all risks, but we will work closely with partners and landowners in affected areas, and we are monitoring biological control trials in other countries, and researching whether these could be used in the UK.

“Meanwhile, we are encouraging sweet chestnut owners to keep an eye on their trees and report any suspected sightings using our Tree Alert pest reporting tool.”

The Forestry Commission is also encouraging anyone visiting or working on woodland sites to practise good biosecurity by not removing twigs, leaves and branches, to avoid accidentally spreading the pest further afield. This includes cleaning clothes, footwear, tools and machinery before moving to other sites.

More detailed biosecurity guidance is available from the Forestry Commission website at

For more on OCGW, including a guide to symptoms, visit

People can report sightings using the Tree Alert on-line information-gathering tool on the Forestry Commission’s website at . Tree Alert helps the Forestry Commission gather information about the health of the nation's trees, woods and forests by reporting signs of tree pests and diseases of concern.

Before reporting a sighting, please check it’s a sweet chestnut tree - there is no need to report galls on other trees - then check the symptoms.

People who cannot use Tree Alert may report sightings by email to (preferred) or by telephone to 0300 067 4000.

Reports of suspected cases in the plant trade should be routed directly to APHA's Plant Health & Seeds Inspectorate helpdesk - tel: 01904 405138; or email: .


  1. In 2015, OCGW was found in Britain for the first time in Farningham Woods in Kent, and in a row of trees in a street in St Albans, Hertfordshire. All the sweet chestnut trees in the St Albans street were destroyed. An area of trees in the most-affected part of Farningham Woods was felled to reduce the population and risk of spread, but it was not possible to fell all the affected trees before the adult wasps emerged from the galls and laid their eggs. Many of the trees felled were due for harvesting as part of the coppice management of the woods, and sweet chestnut trees regrow readily from the stools, or stumps.
  2. The chestnut gall wasp is a native of Asia, and was probably accidentally introduced to Europe in trade. It is not known exactly how it entered Britain, but investigations by the plant health authorities have not found any evidence that it had entered with plants imported for planting schemes.
  3. The ‘galls’ are bulbous growths which the insect’s larvae form on buds, leaves and petioles (leaf stalks). Severe attacks can cause trees to go into decline and reduce nut production, and there is some evidence that they can affect timber quality.
  4. The UK has European Union Protected Zone Status against Oriental chestnut gall wasp. Pending landings of sweet chestnut plants being imported into Britain must be pre-notified to the plant health authorities to enable them to be inspected. This regulation was introduced primarily to protect against the accidental introduction of Cryphonectria parasitica, a fungus which causes sweet chestnut blight, but it also helps to protect against OCGW.
  5. The galls are formed in the spring, and become most easily visible in the summer.
  6. Oriental chestnut gall wasp’s scientific name is Dryocosmus kuriphilus.
  7. Further information is also available from Observatree, a tree health early-warning initiative. Led by Forest Research, the Observatree partnership’s network of 200 trained volunteers support government tree health officers and scientists by surveying trees and helping to process and verify tree health reports received. See for more details.