The biology and immature stages of Thrybius togashii Kusigemati (Hymenoptera: Ichneumonidae: Cryptinae), with a description of the male

Thrybius togashii Kusigemati is found to be an ectoparasitoid of larvae of a phytophagous eurytomid, Tetramesa sp. (Hymenoptera), growing gregariously in internodal cavities of reeds (Phragmites japonica Steud.), in Japan. The female of T. togashii oviposits into reed canes and its predaceous larva consumes almost all eurytomid larvae in an internodal cavity. This species is bivoltine. The mode of parasitism of this species is unique in some respects. First, when the female oviposits, no food resources are available in the reed. Second, the parasitoid allows part of the host larvae to grow and exploits increasing food resources. The egg, pupa and first to final instar larvae and adult male are described and figured for the first time. Colour dimorphism in both sexes and adult morphology adapted to habitat and ovipositional substrate are also discussed.