A Knopper Gall

This is a Knopper gall on an acorn

Andricus quercuscalicis is a gall wasp species inducing Knopper galls.

Knopper galls develop as a chemically induced distortion of growing acorns on Pedunculate Oak (Quercus robur) 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, stems, etc.

The Turkey Oak (Quercus cerris), introduced into Britain in 1735, is required for the completion of the life cycle of the gall.

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