A rangeland grasshopper model (HopMod) was developed to incorporate phenological development, forage destruction, natural mortality, and control option efficacy (of the current registered treatments) for a a complex of rangeland grasshopper species. These factors must be considered to make economically based management decisions for rangeland grasshoppers. The phenology component is based on heat accumulation for populations under field conditions. Phenology parameters were developed directly from field data from Montana over many sites and years for the species complexes of interest. In addition, HopMod phenology and density can be initialized with real–time field data to improve accuracy. Therefore, the model has potential application over broad geographic areas where similar species complexes occur. This is important because it is very expensive and impractical to collect data necessary to develop site–specific models over such large areas in the western United States. In addition to phenology, there are components to simulate forage consumption, oviposition, natural mortality, and control mortality (including chemical and biological control). The population dynamics component was validated with field data from western North Dakota. Forage consumption was verified by comparing HopMod with previously described methods to estimate grasshopper forage consumption. HopMod was designed to run on a microcomputer (MS–DOS, written in Borland Pascal) as part of a larger decision support system, but can also run as a stand–alone program. The decision support system uses results from HopMod to determine economic utility of treatments suggested by an expert system.