Energetic components (respiration, consumption, frass production, and growth) for larvae of the western spruce budworm (WSB), Choristoneura occidentalis, were modeled using multiple regression techniques for a range of temperatures (10-31°C) and body weights (5-200 mg). These functions were used in a simulation of the bioenergetics of the WSB under different variable temperature regimes (average temperatures ranging from 10 to 22°C). Simulation results showed that production increased as temperature increased. Net production efficiencies remained high (maximum ca. 32%) for all temperature regimes save the lowest, in which the maximum net production efficiency (production/assimilation) was 16%. Final assimilation efficiencies ranged from 50 to 52%. Early instar larvae had low total respiration costs, high assimilation efficiencies, low consumption rates, and rapid rates of tissue production. As the larvae increased in size, consumption rates increased, assimilation efficiencies declined, yet the total amount of energy assimilated increased so that production continued. Population energetics of the larvae showed that WSB were similar to other herbivorous ectotherms. Relative to endothenns, ectotherm populations consume similar quantities of energy and exist at higher biomasses per unit area.