Sympatric insect species that do not share sex pheromone components but have a common host and overlapping adult flight periods are potential targets for the development of a combined sex pheromone-based monitoring tool. A system using a single synthetic pheromone blend in a single lure to bait a single trap to monitor two pests simultaneously represents a novel approach. In this study, a combined pheromone-based monitoring system was developed for two lepidopterous defoliators of trembling aspen Populus tremuloides Michenaux in western Canada, Malacosoma disstria Hubner (Lepidoptera: Lasoicampidae) and Choristoneura conflictana (Walker) (Lepidoptera: Tortricidae). Efficacy and longevity of a lure containing both species' pheromones were tested. Immature stages of each species were sampled to evaluate the ability of pheromone traps baited with the combined lure to predict population density. The combined lure was as attractive to M. disstria and C. conflictana males as were traps baited with each species' pheromone alone. Lure age had no effect on attraction of male C. conflictana to the combined lure but had a negative effect on attraction of M. disstria. The number of male moths captured in traps baited with the combined lure was related to immature counts for both species. Pupal counts of M. disstria and larval counts of C. conflictana provided the best relationships with male captures. The combined lure does not attract M. disstria males in direct proportion to population density, because trap catch was comparatively low at high-density M. disstria sites.