Oriental Chestnut Gall Wasp

Oriental chestnut gall wasp was first recorded in the UK from a site in Kent in 2015. Shortly after a second site was discovered and reported by an Observatree volunteer in Greater London, demonstrating that it was present beyond the original discovery site.  

The black, 2.5-3mm long, seldom seen adult wasp is native to Asia and its presence can be easily determined in the UK Springtime when green or rose-coloured galls appear on the buds, leaves or stalks of their host trees, Sweet chestnut (Castanea sativa). The Oriental chestnut gall wasp is the only known gall producing organism on sweet chestnut and is considered an important pest worldwide.   

The bulbous galls it creates may reach 40mm in diameter and a tree with a severe infestation may be greatly weakened making it more susceptible to other pests and diseases, such as sweet chestnut blight which can be fatal. Once galls have been vacated by the new adult wasps they may remain on the trees as woody, dried galls for several years.  

Since 2015 the population of the wasp has spread to neighbouring counties, but in 2021 the legal release of the biological control agent Torymus sinensis was started. This small parasitoid wasp preys upon on the Oriental chestnut gall wasp and it is steadily following its host’s population spread. 

For more information about Oriental chestnut gall wasp, see the resources below.