Oriental chestnut gall wasp – an update

Oriental chestnut gall wasp (OCGW) (Dryocosmus kuriphilus) is the most important insect pest globally of sweet chestnut (Castanea spp.).

It is native to China and has been accidently introduced to other parts of Asia, North America and Europe.

It is a regulated pest in the European Union.

What has happened with OCGW in the UK since it was first found in 2015?

The number of locations where OCGW has been found has risen dramatically from two in 2015 to more than 80 in 2018, all in SE England (see the Forestry Commission confirmed distribution map). I have been visiting woods in Kent, where OCGW was first discovered in the UK, annually to monitor the pest and collect samples. There has been a huge increase in the density of galls on individual trees.

Why the concern in the UK?

Sweet chestnut is a valuable timber species and is locally important in Britain, particularly in Kent, where the chestnut coppicing industry has been enjoying a revival in recent years. While I was recently in Kent I saw coppiced-chestnut logs being sawn and expertly split by hand to make fence posts. The wasps can gall the leading apical buds resulting in lateral branching and lowering the value of the coppiced timber. High numbers of galls may weaken trees and make them more vulnerable to other pests and diseases, especially sweet chestnut blight.

What can be done to manage the invader?

Fera Science Limited in collaboration with the University of Edinburgh, Centre of Ecology and Hydrology and other institutions have been researching ways to manage the gall wasp. We have concluded that the only effective method is biological control, that is, to introduce a non-native parasitic wasp (Torymus sinensis) to attack the gall wasp.

The biological control agent has been successfully used in several other countries in Europe and we are gathering evidence to determine whether it would be safe to release the parasitic wasp in the UK.

As at June 2018, an application for a license from Defra to release the biological control agent is currently under review.

This is a complicated analysis. For example, one ‘fly (or wasp) in the ointment’ is that we have native parasitoid wasps that can attack both the gall wasp and the biological control wasp. Parasitic wasps that attack other parasitic wasps are called ‘hyperparasites’. As Augustus De Morgan (1872) wrote:

Great fleas have little fleas upon their backs to bite ’em,
And little fleas have lesser fleas, and so ad infinitum.

An update on OCGW should not forget to include mention of one of the great Observatree success stories – that in 2015, a volunteer (Amanda Yorwerth) found the second site of OCGW in the UK. Observatree volunteers are continually reporting on both positive and negative sightings in SE England.

If you suspect you have seen a sighting of OCGW then please4 report this to Tree Alert as soon as possible. To help you confirm your sighting you can refer to our free OCGW Field ID Guide which illustrates signs and symptoms.