The economic injury level (ElL) model traditionally used in pest management was extended to include control of univoltine pests for which multiple years of control benefits are expected: The model was then applied to control of grasshoppers (Acrididae) invading western rangelands. The ElL for grasshoppers was 18 fourth instars per square meter under defined average conditions using malathion ULV liquid spray, but this threshold varied greatly depending on specification of expected forage value, treatment costs, and other model parameters. The ElL depended on several key factors including the value of forage, the cost of treatment, pest population dynamics between years, the life of the treatment, and treatment efficacy. Other factors that were less important in determining the ElL were the discount rate used in discounting forage savings benefits in future years and stage of pest development when treated. Uncertainty about site-specific specification of model parameters, as well as site-specific species complexes, survival rates, and forage destruction rates, suggests a large degree of uncertainty associated with the ElL.