Adult rangeland grasshoppers,Aulocara elliotti(Thomas), from 2 geographically isolated wild populations were maintained at warm (24° night and 30°C day) or cool (18° night and 24°C day) temperatures and fed Western wheatgrass (Agropyron smithiiRydb.) grown in a greenhouse insectary at the same warm or cool temperatures or collected from the field where diurnal temperatures fluctuated widely during the rearing season (0°–34°C). Not only were fecundity, egg viability, and female longevity significantly affected by insect rearing temperatures, but the growth temperatures of the host plant also caused significant variation in these characteristics. Grass grown at cool temperatures increased the rate of egg production and decreased the avg female longevity. Females fed field grass had the lowest rates of egg production and lived the longest, while those fed grass grown at warm temperatures had intermediate rates of egg production and avg longevities. The effect of the host plant on reproduction was modified, however, by the temperature at which the insect was reared and by other factors within the populations. These results may provide a basis for evaluating the fluctuations in natural populations of grasshoppers.