Bluestem Gall Midge Annual Production Cycle and Effects on Grass Seed Production

The bluestem gall midge (Stenodiplosis wattsii Gagne) is native to the grasslands of North America. It feeds on the developing seeds of warm-season grasses during a portion of its lifecycle, but little is known of the biology and extent of gall midge infestations in native warm-season grasses in the USA. We investigated the infestations of the bluestem gall midge in big bluestem (Andropogon gerardii Vitman), sand bluestem (Andropogon gerardii var paucipilus), little bluestem [Schizachyrium scoparium (Michx.) Nash], and indiangrass [Sorghastrum nutans (L.) Nash] commercial seed production fields during 2001 through 2004. Our objectives were to determine the annual production cycle of the bluestem gall midge, the range of the infestation in Nebraska, and estimate its impact on seed production. In eastern Nebraska, the midge goes through four generations with the fourth entering a larval diapause, overwintering in seeds and emerging in mid-June of the following year. The bluestem gall midge damaged from 31% of the little bluestem seed to 67% of the big bluestem seed, and, on average, reduced the production of viable seed by 54% across the four grass species in eastern Nebraska. Additionally, the warm-season prairie grasses were surveyed in nine other counties in Nebraska and the bluestem gall midge was found in each species surveyed from all nine counties. This study is the first to document the negative effects of the bluestem gall midge on the production of viable seed in sand bluestem and indiangrass.