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Coelioxys bisoncornua, one of the largest Coelioxys species in North America, was first described by Roscoe E. Hill in 1936 based on three males and two females collected in Nebraska and Kansas (Hill 1936); additional records have since been published for Minnesota and Illinois as well (Baker 1975; Bouseman 1982). This species appears to be closely associated with Helianthus sunflowers. Like other Coelioxys bees, C. bisoncornua is presumably a cleptoparasite. Rather than collect pollen and provision a nest for their young, cleptoparasitic bees instead deposit their eggs in the nests of other bees. The host of C. bisoncornua is unknown, but based on circumstantial evidence Bouseman (1982) proposed Megachile (Phaenosarus) fortis as a likely candidate.