I am highlighting the problem of the Sweet Chestnut (Castanea) Gall Wasp or “Dryocosmus kuriphilus”. This pest has had, I believe a few occurances in the UK. But it was detected for the first time in Spain in 2012, the species spread very quickly and its is now present in extensive areas of the distribution area of Castanea sativa in the Iberian peninsula. Today, it is threatening the rich chestnut fruit production of areas of Galicia and Andalucía, where heavy infestations have been detected.
It is a native species in China, from it was accidently introduced. It has many localised pockets in Italy and France, and it is caused significant drops in nut production.
The wasp lays its eggs in the buds, during autumn. Then when the leaves begin to unfurl in the spring, the eggs hatch, which then turn into larvae (the ‘grubs’, or immature life stage) causes abnormal growths, called galls, to form on the buds, leaves, and petioles (leaf stalks).
These galls or blisters are green turning a reddish colour in June, before the gall wasp emerges. The leaves tend to get distorted, as can be seen in the first photo.
High numbers of 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