(Z,Z,Z)-1,3,6,9-Nonadecatetraene is a sex pheromone that attracts males of both the winter moth Operophtera brumata (L.) and the Bruce spanworm O. bruceata (Hulst). Tests were run to maximize the specificity of a pheromone trap for the winter moth by the addition of a previously discovered Bruce spanworm male inhibitor (BSMI), (E,Z,Z)-1,3,6,9-nonadecatetraene. Trap capture inhibition of O. bruceata would facilitate monitoring of winter moth as males of the two species are difficult to distinguish without dissection. Wind tunnel tests with O. bruceata males responding to the pheromone indicated that BSMI has a more potent inhibitory effect if males physically contact the compound. Field tests in an area where only O. bruceata was found demonstrated that pheromone-baited traps caught fewer O. bruceata with BSMI placed on the outside of the entrance holes than when it was placed on the inside of the trap, catching respectively 97 and 82% fewer males than traps baited with the pheromone alone. In an area where O. brumata predominated, BSMI, whether inside or outside the trap, did not affect O. brumata captures. However, O. brumata trap captures were reduced when BSMI was placed on rubber rings glued to the outside of the entrance holes to the traps even though control rings did not affect O. brumata captures. Evidence is presented indicating that hybridization is taking place between these two species where O. brumata has recently been introduced and that the response of the hybrids to the BSMI is intermediate between the two species.