Hairy Beech Gall

Hartigiola annulipes is a midge that causes galls to form on the leaves of beech. Young galls develop early in the season as a small circular pustule or flattened dome which is visible on both leaf surfaces. Mature gall is cylindrical, hairy or smooth and up to 6 mm high when mature in late August or September. The gall is pale green and any hairs either pale or reddish brown. The larva is white. The gall is seen more often than the midge.

Source: http://www.naturespot.org.uk/species/hairy-beech-gall