Technical Abstract: Infocrop, a generic crop growth model was used to simulate the effect of Russian wheat aphid, Diuraphis noxia (Mordvilko) damage on winter wheat at Fort Collins and Akron, Colorado. Observed and simulated yield reductions in four experiments over a period of two years were found to be closely related (R2= 0.85). The aphid damage mechanisms coupled to the crop growth model could thus be validated through field experimental data. Economic injury levels for Russian wheat aphids determined with the validated model revealed that winter wheat was more prone to aphid attack during early growth stages than during late tillering and heading. Economic injury level changed among years and were directly related to cost of control but inversely related to market value of winter wheat. Infocrop and GPFARM were used to simulate effect of downy brome weed, Bromus tectorum L. at Hays (Kansas) and Cheyenne (Wyoming) and jointed goat grass, Aegilops cylindrica Host at Archer (Wyoming) on winter wheat. Both models simulated the effect of downy brome on winter wheat well. The average observed and simulated yield reductions with Infocrop over a period of three years were closely related (R2=0.941). The effect of jointed goat grass on winter wheat was simulated appropriately by GPFARM but not by Infocrop for want of required data on relative weed cover. Validated simulation models can be used for various applications such as for establishing economic thresholds and devising iso-loss curves for the pests. Simulation models have a great potential as guides for optimal pesticide use.