Knopper gall on acorns - healthy and infested acorns in proximity - stock photo

Knopper galls were first seen in England on oak trees in 1960 and are now widespread. At first it was feared that this would endanger the native oaks but this has not been shown to be a cause for concern and they are now regarded as part of the life inhabiting oak trees. The galls are caused by gall wasps laying eggs in the buds and this results in conical deformity of the acorn.