Andricus aries is a species of gall-forming wasp, in the genus Andricus. The adults lay their eggs on various species of oak, including Quercus robur, Q. petraea, Q. pubescens and Q. cerris, and the developing larvae cause the trees to create an elongated gall, reminiscent of a ram's horn, hence the epithet aries and the vernacular name "Ram's Horn Gall Wasp".