The forest tent caterpillar, Malacosoma disstria (Lepidoptera: Lasiocampidae) and large aspen tortrix, Choristoneura conflictana (Lepidoptera: Tortricidae) are both important defoliators of trembling aspen in Canada. As these species do not share pheromone components but do exhibit overlapping adult flight periods, the opportunity exists to develop a combined sex pheromone-based monitoring tool to detect temporal and spatial changes in population density. Pheromones for both species were combined in rubber septa lures and compared to lures baited with each species' pheromone alone for attractiveness to adult males. Traps baited with the combined lure were hung at varying population densities of each species in 2005 and 2006 in Alberta, Canada. Immature stages of each species were sampled both years to test the ability of the model to predict their relative densities. The combined lure is as effective in attracting adult males as the individual pheromone alone. The efficacy of the predictive power of the monitoring tool will be presented.