Technical Abstract: Little is known about the ecological processes that generate grasshopper outbreaks, and the complex ecological interactions between grasshoppers, weather conditions and plants that cause fluctuations in grasshopper populations remain poorly understood. The effects of initial and increasing grasshopper densities on grasshopper population dynamics, reproductive correlates and rangeland vegetation were examined at a northern mixed-grass prairie site highly dominated by western wheatgrass though manipulations of grasshopper densities inside 10 m2 cages. High quality grass growth occurred following a large late summer precipitation event of 9 cm. Reduced proportional survival was apparent in the two higher density treatments prior to the rain, indicative of food limited density dependent mortality. However, the late summer rainfall event mediated the effects of high grasshopper densities on demographic characteristics due to the growth of young higher quality vegetation in late summer. This led to weak effects of competition on survival and reproduction at the end of the experiment even in the increased density treatment where densities were above 30 per m2. A severe grasshopper outbreak occurred in the following year, with nymphal densities reaching 130 per m2. As a result, the origin of a severe grasshopper outbreak can be traced to the availability of high quality grass biomass following late summer rain. In years with lower late summer precipitation, stronger and more consistent density dependent effects would be expected with similar grasshopper densities.