Spatial displacement of release point can enhance activity of an attractant pheromone synergist of a bark beetle.
Flight responses of the southern pine beetle, Dendroctonus frontalis Zimmermann, to widely-spaced (>130 m) traps baited with pine volatiles (in turpentine) and the female-produced pheromone component frontalin were enhanced when a bait containing the male pheromone component (+)-endo-brevicomin was attached directly to the trap. However, displacing this bait 4–16 m horizontally from the trap significantly increased its synergistic effect. (+)-endo-Brevicomin enhanced catch to the same degree when the bait was positioned either on the trap or 32 m away. In another experiment, pairs of frontalin/turpentinebaited traps were established with 4 m spacing between traps and >100 m spacing between pairs. Attachment of either a racemic or (+)-endo-brevicomin bait to one trap of a pair caused a significant increase in catch by both traps, but catch in the trap lacking endo-brevicomin was increased more than in its endo-brevicomin-baited twin. In a third experiment, widely-spaced groups of three traps (in a line with 1 and 4 m spacing between the middle and outer traps) were baited uniformly with frontalin and turpentine, and the release rate of (+)-endo-brevicomin from the middle trap was varied across three orders of magnitude. Release rates sufficient to enhance total D. frontalis catch by the trio also caused relatively higher catches to occur in the outer traps than in the middle one. These experiments indicated that both male and female D. frontalis fly to and land preferentially at sources of frontalin and host odors when these are located some distance away from a source of endo-brevicomin. This behavior may have evolved in D. frontalis to allow host-seeking beetles to locate growing, multi-tree infestations while avoiding fully-colonized trees within these infestations. Our data demonstrate that trap spacing alone can qualitatively change the outcome of bait evaluation trials and may explain why many earlier experiments with endo-brevicomin failed to identify it as an aggregation pheromone synergist for D. frontalis. We believe that important aggregative functions of semiochemicals of other bark beetle species may have been similarly overlooked due to choice of experimental procedures.