Computer simulation studies reported here examine potential for injury to alfalfa,Medicago sativaL., byAgromyza frontella(Rondani), the alfalfa blotch leafminer (ABL). A validated, physiological model of alfalfa growth, ALSIM 1 (Level 2), was modified to incorporate eight hypothesized mechanisms of injury by ABL. A phenological model forecasting occurrence of damaging stages of ABL was used to time simulated injury under nine weather regimes. Sensitivity of yield, quality (leaf fraction), and end-of-season root reserves to different types and degrees of injury were examined. Leaf mining by third instars had virtually no effect on yield during any harvest period under any weather conditions. Leaf fraction of harvested hay increased slightly. There was essentially no impact on end-of-season root reserves. Model results were insensitive to major qualitative changes in the way larval injury was represented. Simulated pinholing injury by adult female ABL had a greater effect. Greatest reduction in total yield (15%) occurred when pinholing was combined with low rainfall. Pinholing also significantly increased the leaf fraction of harvested hay but had little or no influence on end-of-season root reserves. Sensitivity analyses showed that ALSIM was more responsive to all types of damage during the period of ABL adult activity than during the period of larval mining. The model was most sensitive to changes in rates of photosynthesis and assimilate accumulation, and to a lesser degree to changes in level of soil moisture at which water stress was initiated and rate of evaporation from the canopy. During the period of ABL activity, the model was fairly insensitive to alteration of light absorption by the canopy and insensitive to changes in leaf area index and senescence rate of leaves. Cutting management (scheduling) was examined as a factor influencing simulated impact of ABL on alfalfa. Early harvest most often reduced percentage yield loss caused by ABL and reduced the already small effect of ABL on end-of-season root reserves.