The fecundity and fertility of parent adult western spruce budworm, Choristoneura occidentalis Freeman, which survived treatment with IGRs in their last larval stage was assessed. With few exceptions, IGR treatment significantly reduced both reproductive factors regardless of whether the male or female parent had been treated. A simple model estimating population reduction in the F1-generation based on lethal and reproductive effects on the parents is described. If IGRs were applied at a dosage of 154 g/ha (ca. 2 oz/acre), the estimated percentages of F1 reduction which would result ranged from 74.6 (ZR-515) to 99+ (ZR-512 and Ro 10-3108 (oil)).