The mountain pine beetle (MPB), Dendroctonus ponderosae (Coleoptera: Scolytidae), is broadly distributed in North America, ranging from British Columbia south into the Rocky Mountain and Cascade/Sierra Nevada regions.  It is one of the most economically significant forest insects in the world.  The commercially available synthetic pheromone lure for this insect is based largely on research conducted in British Columbia and the Pacific Northwest.  There has been speculation that the standard formulation of the synthetic pheromone [50%-(–)-exo-brevicomin, 86%-(–)-trans-verbenol, and myrcene] may not be the most efficacious blend for the Rocky Mountain population.  In some cases, beetles have completely ignored baited traps and attacked adjacent, unattacked (i.e. relatively unattractive) hosts.  Future application of pheromones as a MPB population management tool should be cognizant of geographic variation in chemically-mediated aggregation behavior among populations of the beetles rather than just the impact of host species.  Field studies in ponderosa pine and lodgepole pine stands at four locations in the Rocky Mountains (South Dakota, Colorado, Idaho, and Arizona) have demonstrated that replacing myrcene with terpinolene increases the response of the MPB by 7- to 88-fold, depending on the site.  The South Dakota population was different from the others in that, although it still preferred terpinolene, it also responded to other monoterpenes at a relatively high rate.  The overall sex ratios (female:male) of MPB at each site were all female-biased ranging from 2.1:1 to 9.2:1.  This suggests that a female-produced component of the attractant is yet to be identified from the Rocky Mountain populations of MPB.