Populations of Dendroctonus ponderosae Hopk. aggregate on pine trees in response to a combination of host- and insect-produced volatiles. Release of the insect-generated volatiles by defecation coincides with the release of host volatiles as resin exudes from the galleries initiated by the female beetles. When tested with oleoresin under field conditions, synthetic trans-verbenol, the major insect pheromone, was highly attractive to both sexes of D. ponderosae. Also, emergent female Dendroctonus brevicomis Lec. and Dendroctonus frontalis Zimm. crushed to powder at −70 °C were attractive; emergent females of both species are known to contain large amounts of trans-verbenol. Trans-verbenol per se, however, was not attractive to walking or flying D. ponderosae.