Effects of weather on infestation levels of grasshopper assemblages in the shortgrass prairie regions of Montana, Wyoming, Colorado, and New Mexico were examined using correlation and multiple regression analyses. Fifty-three years of population and monthly weather data were analyzed to estimate the effects of temperature, precipitation, and a heat/precipitation (H/P) drought index on extent of infestation of grasshoppers. Regressions and correlations indicated that effects of weather varied with latitude. Infestation levels in both northern states (Montana and Wyoming) were positively associated with 3-yr mean July-August temperatures. Significant terms in regressions for the southern states (Colorado and New Mexico) were spring and summer H/P ratios; infestation was negatively associated with H/P. These results suggest that grasshoppers in more northern states are favored by warm, dry summer conditions, whereas grasshoppers of southern areas appear to require spring and summer moisture. Results are compared with similar analyses done for grasshoppers in Canada and for Old World locust populations. Impact of insecticides on long term infestation trends was analyzed using 25–30 yr of grasshopper suppression program data. There was little evidence that grasshopper suppression activities interfered with our perception of weather–population relationships. Regression analyses indicated that number of hectares sprayed the previous year was significantly and negatively associated with infestation in Colorado and New Mexico.