Male and female Monochamus clamator (LeConte) and M. scutellatus (Say) are able to detect bark beetle pheromone components electrophysiologically and are attracted to traps baited with blends of pheromone components of scolytid bark beetles. We investigated the effect of individual pheromone components (ipsenol, ipsdienol, 3-methyl-2-cyclohexen-1-one (MCH), frontalin, verbenone, cis- and trans-verbenol, and endo- and exo-brevicomin) on Monochamus Dejean trap catches. Only traps baited with ipsenol and/or ipsdienol together with the host volatiles ethanol and α-pinene caught significantly more male and female M. scutellatus and M. clamator than traps baited with host volatiles alone. Ipsenol and ipsdienol are aggregation pheromones of secondary bark beetles in the genus Ips DeGeer while the other components are pheromones of primary bark beetles in the genus Dendroctonus Erichson. The former should be the most reliable indicators of suitable host material because most Ips spp. attack weakened or moribund trees or trees already successfully under attack by primary bark beetles, and their pheromones may be more persistent in space and time than those of Dendroctonus spp. In two successive years in an operational (commercial) mass-trapping program, traps baited with ethanol, α-pinene, and ipsenol captured twice as many beetles as traps baited with host volatiles alone. These results suggest that operational monitoring or mass-trapping programs could be improved significantly by the inclusion of ipsenol in baits at a minimal cost.