Selection imposed by mating disruption could lead to changes in the pest insect’s chemical communication system if there is genetically based variation in signal and/or response characteristics. Ultimately, this evolution, in the absence of courteracting selection, could lead to resistance to this control tactic. For this reason, the effects of mating disruption on the reproductive success of two pheromone strains of the cabbage looper, Trichoplusia ni (Hübner), were examined in field-cage experiments in which we could monitor mating success. The first experiment evaluated mating disruption using the major component of the cabbage looper pheromone [(Z)-7-dodecenyl acetate: Z7–12:Ac], and the complete pheromone blend emitted by females. The second experiment focused on disruption with the compound that is often most abundant in the pheromone blend of mutant females [(Z)-9-tetradecenyl acetate: Z9-14:Ac], and the complete pheromone blend emitted by mutant females. In the first experiment, mutant males tended to have greater mating success than normal males in the presence of the full normal blend, as they do in the absence of disruptants. By itself, this result would indicate that the mutant alleles would spread through a population. However, normal females had greater mating success than mutant females in the presence of the normal blend disruptant (and Z7-12:Ac). This directional selection on females would tend to counter the selection on males, and dampen or eliminate any change in allelic frequency in the next generation. In the second experiment, the full mutant blend decreased male and female mating frequency relative to control, but did not influence the relative mating success of the two strains. Results of these experiments demonstrate for the first time that the effects of a mating disruptant on mating success of T. ni can be complex, with the pheromone blend having sometimes counteracting effects on pheromone strains and sexes.