Knopper galls on oak

Every single acorn on this young oak tree has been affected by galls.

Knopper galls
Knopper galls

I think that they are knopper galls, caused by the wasp Andricus quercuscalicis’s egg laying. The affected acorns, all of them in this tree’s case, are unable to develop properly or germinate. Instead, they will fall to the floor and next spring female wasps will emerge from them and go looking for Turkey oaks (Quercus cerris): without Turkey oaks, they can’t complete their lifecycle.

knopper galls3

knopper galls 4
The deformed acorns feel slightly sticky and waxy.

There is a fascinating account of the wasps (Andricus quercuscalicis) and their galls on the University of Reading’s biodiversity blog or at Wikipedia.

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