Knopper gall

Andricus quercuscalicis is a gall wasp species inducing knopper galls.
Knopper galls develop as a chemically induced distortion of growing acorns on French oak trees, caused by gall wasps, which lay eggs in buds with their ovipositor. The gall thus produced can greatly reduce the fecundity of the oak host, making this gall potentially more of a threat to the reproductive ability of the tree than those that develop on leaves, buds or stems. The Turkey oak is required for the completion of the life cycle of the gall. The large 2 cm galls are sticky and red, later becoming woody and brown.