Knopper galls develop as a chemically induced distortion of growing acorns on pedunculate oak trees, and the gall produced didn't greatly affect the fecundity of the oak host, as much as it could if it develops on roots or stems.
Andricus quercuscalicis is a gall wasp species inducing knopper galls. 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, stems, etc. 
