Seed pests and pathogens are sometimes inadvertently introduced into new areas. For example, Knopper galls are caused by a gall wasp (Andricus quercuscalicis) which may have crossed the English Channel on high altitude winds. It was first recorded in the 1950s but has since spread throughout England, Scotland and Wales. The gall wasp requires both pedunculate oak (Quercus robur) and Turkey oak (Quercus cerris) to complete its obligate 2-phase life cycle. The Knopper galls are due to a chemically induced distortion of the acorns, which results when the gall wasps lay its eggs. It affects crop yield but not enough to warrant control measures.