EFFECT OF HESSIAN FLY (MAYETIOLA-DESTRUCTOR) ON PRODUCTION AND PERSISTENCE OF PRAIRIE GRASS (BROMUS-WILLDENOWII) WHEN ROTATIONALLY GRAZED BY DAIRY-COWS

The growth and persistence of established prairie grass plants and prairie grass seedlings arising from natural reseeding, were measured from March 1987 to April 1989, in a mixed species pasture dominated by prairie grass, and rotationally grazed by dairy cows. The growth and persistence of prairie grass plants and seedlings in areas where attack by insects were reduced by insecticide applications were compared with areas receiving no insecticide from December 1987 to April 1989. Summer/autumn growth rates of the pastures were increased where Hessian fly damage was reduced by insecticide application. The frequency of point quadrat hits on prairie grass was consistently higher in insecticide-treated than in untreated plots. Insecticide treatment slowed the rate of decline in adult prairie grass plant density, but despite recruitment of new plants from natural reseeding during winter 1988, the overall population at the end of the experiment was only 46% of the initial population. Prairie grass seedlings appeared in largest numbers during late autumn/early winter, but losses were high during late spring, summer, and autumn, particularly during grazings. Although insecticide treatment reduced tiller damage as a result of Hessian fly larval feeding, it did not alter the trend for a decline in prairie grass plant and seedling densities. Management practices encouraging natural reseeding in dairy pastures are unlikely to offer a viable method of maintaining prairie grass populations where Hessian fly is present.