Semiochemical background in the environment can influence insect orientation to release points of the same or different semiochemicals. endo-Brevicomin is a pheromone component of the tree-killing bark beetle Dendroctonus frontalis Zimmermann (Coleoptera: Curculionidae: Scolytinae) that has a biphasic dose–response curve, enhancing attraction at low release rates but reducing attraction at high rates. We investigated the effect of artificial manipulation of background levels of endo-brevicomin on D. frontalis responses to sources of aggregation attractant in the field. Traps baited with the aggregation pheromone component frontalin and the host odor alpha-pinene were deployed either with or without a background of endo-brevicomin produced by three surrounding dispensers of this semiochemical each located 20 m away. Two tested levels of endobrevicomin background caused catches to increase by an order of magnitude above those in the absence of background. Presence of background also altered the beetles' biphasic dose–response when endo-brevicomin dispensers were added to traps. Background reduced or concealed attraction-enhancement otherwise observed for low-release dispensers added to traps, and it decreased the release rate necessary to produce reductions in catches. We propose that spatial variability in abundance of natural, background sources of endobrevicomin in the environment (i.e., infested trees) is a cause of the observed variability in effects of endobrevicomin dispensers on southern pine beetle behavior in the field. Furthermore, our results illustrate the potential complexity of the density-dependent effects of biphasic pheromone components on bark beetle mass attack and colonization behavior.