EFFECT OF GRASSHOPPER (ORTHOPTERA, ACRIDIDAE) DENSITY AND PLANT COMPOSITION ON GROWTH AND DESTRUCTION OF GRASSES

A study was conducted to determine the effects of grasshopper density and plant composition on grass growth and destruction by the mixed gra ss- and forb-feeding grasshopper, Melanoplus sanguinipes (F.). Using a completely randomized design, 125 1-m2 cages were placed on plots on mixed-grass rangeland in western South Dakota and stocked with 0, 5, 1 0, 15, or 20 fifth-instar M. sanguinipes. Grasshopper densities within the cages were monitored approximately weekly, and total grasshopper feeding-days were calculated for each cage for 68 d. Before adding gra sshoppers, biomasses of grasses and forbs within the cages were estima ted visually and after 68 d, final biomass of vegetation was determine d. Results from analysis of variance with regression and multiple regr ession analysis indicated that grasshopper density and the initial bio mass of western wheatgrass and shortgrasses within the cages significa ntly affected the final biomass and growth of western wheatgrass and t he amount of grass removed by grasshoppers. However, significant reduc tions in grass biomass occurred only in cages stocked with 15 and 20 i nsects, equivalent to 845 and 1, 112 grasshopper feeding-days, respect ively. Grasshopper density had no significant effect on biomass of sho rtgrasses and Japanese brome, although growth of these species was aff ected by initial biomass of grasses in the cages.