Chestnut Gall Wasp
Chestnut Gall Wasp, Oriental Chestnut Gall Wasp
Dryocosmus kuriphilus, or the Chestnut Gall Wasp, is a tiny, fly-like wasp. Chestnut Gall Wasps are highly specialised, they're sometimes referred to as parasitic wasps or parasitoids. They require a plant host to reproduce successfully. The female gall wasp lays eggs in the leaves of Sweet Chestnut trees, resulting in the production of pink and green fleshy galls that protrude from the veins of leaves. Invasive species can be problematic because native species are not used to their effects. Unfortunately, the presence of this wasp can weaken Sweet Chestnut trees, making them vulnerable diseases like sweet chestnut blight. Luckily this wasp isn't found in South Africa.
These wasps are invasive.
Adults: These wasps are so tiny it's unlikely you'd see them. Their presence is typically recognised by the galls produced as a result of the larval activity in plants. The gall wasp eggs and larvae remain inside the plant and gall. Dryocosmus kuriphilus is the only species that will use sweet chestnut to lay eggs. If galls are sighted in sweet chestnut trees, then it should be reported via TreeAlert.
Abnormal protrusions, or galls, will be evident on the leaves and branches of sweet chestnut. Infested leaves may fall prematurely.
Northern America, Asia and Europe
Unfortunately, plants can't be treated for these insects. The problem with gall-forming insects is that they're concealed and protected within the plant gall. A parasitoid wasp is being trialled as a form of biocontrol in some parts of Europe. However, this is hasn't been tested in the UK.
Insecticides are unlikely to work on this wasp. Any sightings should be reported via TreeAlert.