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Andricus quercuscalicis is a gall wasp species inducing knopper galls.
Knopper galls develop as a chemically induced distortion of growing on pedunculate oak ( Quercus robur L.) trees, caused by , 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 word knopper derives from the German word 'Knoppe', meaning a kind of felt cap or helmet worn during the 17th century;Tait, Norman & Pearl. The Spread of Knopper Gall Wasps into the Clyde area. Glasgow Naturalist 2004, Vol. 24, pp. 131–132. also a small rounded protuberance, often decorative, such as a stud, a tassel or a knob. The origin of the name.
Although normally distinctive the knopper gall can, under some growth conditions, be mistaken for the acorn cup gall, caused by the gall wasp Andricus grossulariae.
The first phase, occurs in spring in small conical galls that form on the male catkins of the Turkey oak. Royal Horticultural Society website. During this phase, females lay eggs without mating, and those eggs will produce both male and female wasps (parthenogenesis). This bisexual generation mates, and the females lay their eggs in autumn. This second phase, called agamic generation, occurs in autumn. This phase occurs on pedunculate oaks. The galls created during this phase are commonly known as knopper galls.
The records of the expansion of A. quercuscalicis date as far as 1631, when it was recorded in eastern Germany. Great Britain and Ireland are the most recent territory gains of the species.
In 1979, A. quercuscalicis underwent a population explosion in England and for a time there was concern that it would seriously affect acorn fertility and thus the future of England's most iconic tree. This has not been the case, and control is regarded as unnecessary. Details of its spread to the UK. Knopper galls were first noted at Canonbie in southern Scotland in 1995 and their distribution is often restricted to old country and urban estates where the Turkey oak has been previously planted.
The abnormal acorns develop during summer and the acorn is either wholly or partially replaced by the gall growth. The knoppers become woody and brown in early autumn, after which they fall from the tree and the adult sexual female gall wasp emerges through a vent in the top of the gall in spring. The level of attack by the insect varies greatly from year to year.
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