Andricus infectorius

The asexual generation is a 10–20 mm spherical gall, found from late-May through to October when it matures. Found on the terminal bud of a branch, it is initially green and as it matures darkens to dark brown; falling to the ground in the winter.[1][2] The gall is found on Hungarian oak (Quercus frainetto), sessile oak (Quercus petraea), downy oak (Quercus pubescens) and pedunculate oak (Quercus robur).[2]

Oak gall wasps often have two generations per cycle, with one sexual and one asexual; each creating different galls. According to Milan Zúbrik et al (2013), the sexual generation of the gall is unknown, but likely to be on Turkey oak (Quercus cerris).[1] A very small, ovoid gall on the catkins of Turkey oak may be the sexual generation.[2]

This species has been found in some western and central European countries but is absent from Belgium, Germany, Spain and Switzerland.[3] It has recently been found in Great Britain. A 2013 gall found on Trengwainton Carn, near Penzance, Cornwall was initially identified as Andricus inflator. According to Dr C Leach of the British Plant Gall Society, it is more likely to be A. infectorius.[4]