The asexual generation galls are formed on oak buds from the previous year.  The remains of bud scales at the base, but these may wear off.   Longer than wide, acorn- or egg-shaped, up to 8mm long, and with a blunt tip.  Green and smooth at first with paler spots and streaks, becoming purplish-brown.  Mature galls brown and ridged lengthwise.   There is a single gall-wasp larval chamber, but multiple chambers are made by inquilines. 

Photographs of the gall on the oak twig

oak trees

May to June

uncommon (British Plant Galls, Redfern & Shirley, 2023)

Rare/under-recorded

see species page on Bladmineerders