The Knopper Gall is caused by a tiny gall wasp, Andricus quercuscalicis. It produces ridged outgrowths on the acorns of our native Pedunculate Oak; forming in August they are sticky and red, later becoming woody and brown. A second generation then develops in the catkins of Turkey Oak.
How to identify
Unmistakeable: the Knopper Gall is a nobbly, bowl-shaped protrusion from acorns.