Knopper

A Knoppergall is an abnormal growth or gall induced on acorns of the pedunculate oak ( Quercus robur) by the gall wasp Andricus quercuscalicis. The gall develops as a chemically induced distortion of the acorn and bud tissues.

Knoppergalls vary considerably in shape and size. They typically start as a greenish-yellow, sticky swelling on the developing acorn. As they mature, they become harder, often developing ridges and a more brownish coloration. The shape can be highly variable, ranging from knobbly and rounded to more elongated and distorted forms, sometimes completely engulfing the acorn. The name "Knopper" is believed to be derived from an Old English word related to a type of hat or button, alluding to the gall's shape.

The life cycle of Andricus quercuscalicis is complex, involving alternating generations. The generation that induces the Knoppergall on oak acorns is asexual (parthenogenetic). Female wasps emerge from these galls and lay eggs on the catkins of Turkey oak (Quercus cerris). These eggs produce a sexual generation of wasps. The offspring from these wasps then induce galls on Turkey oak buds. Finally, the female wasps emerging from these Turkey oak galls return to the pedunculate oak, laying eggs in the developing acorns and beginning the Knoppergall cycle anew.

The presence of Knoppergalls can significantly impact acorn production. Heavily infested trees may produce fewer viable acorns, potentially affecting oak regeneration. The galls can also distort the acorn's shape, making it more difficult for animals to disperse the seeds. While Knoppergalls are generally considered a nuisance, they rarely kill the oak trees themselves.

Knoppergalls are a relatively recent phenomenon in some regions, as the Turkey oak, which is essential for the wasp's life cycle, was only introduced to certain areas within the last few centuries. The spread of the Turkey oak has facilitated the expansion of Andricus quercuscalicis and the increased occurrence of Knoppergalls on pedunculate oaks.