Constructing a dynamic model of growth in fir stands (age 30–70 years), based on gross physiological structure of the stand, made possible reasonable qualitative and quantitative forecasts of growth loss due to defoliation by insects in a wide variety of situations. Analysis of the model, in comparison with the natural world, indicates that the key to understanding growth loss in fir stands subjected to defoliation by spruce budworm lies in more explicit knowledge of the mechanisms of new foliage production.