Oriental chestnut gall wasp, Dryocosmus kuriphilus, is an insect of Asian origin which affects sweet chestnut trees in the Castanea family of trees. The only species of the Castanea family grown in significant numbers in Britain is the European sweet chestnut (Castanea sativa).
The wasp is a pest of sweet chestnut trees because activity by its larvae (the 'grubs', or immature life stage) causes abnormal growths, called galls, to form on the buds, leaves, and petioles (leaf stalks). The presence of these galls is the most obvious symptom of infestation. The galls start out green when they develop at bud burst about March, and turn rose-coloured or red by June. They then gradually dry out and turn brown and woody over the summer, when the adult wasps emerge from them, and they can cause the leaves to drop early. Most galls fall off the tree when the leaves fall, although galls attached to the bases of petioles can stay on the tree for some years. No other organism is known to cause galls on sweet chestnut trees, so their presence is a reliable indicator of the pest.
Early galls around March
Later galls in June
On its own the wasp is a low-impact pest of sweet chestnut trees, however, in high numbers the galls can weaken trees and make them more vulnerable to other pests and diseases, especially sweet chestnut blight, which is caused by the fungus Cryphonectria parasitica. Sweet chestnut blight is not known to occur in the UK Severe attacks can result in tree decline.
The wasp does not bite, sting or pose any other threat to people, pets or livestock. It does not attack horse chestnut, or 'conker' trees, which belong to the Aesculus family, or any other species of tree widely grown in Britain.