Unraveling semiochemical signals: Pheromone and host-volatile interactions in eastern Canadian Dendroctonus spp.

Abstract

Bark Beetles (Curculionidae: Scolytinae) are important disturbance agents of coniferous forests that use aggregation pheromones to colonize hosts. Sympatry among bark beetle species results in a complex semiochemical environment in which aggregation pheromones must facilitate mass-attack while minimizing interference between closely related species. The spruce beetle, Dendroctonus rufipennis Kirby (Coleoptera: Curculionidae), and the eastern larch beetle, Dendroctonus simplex LeConte (Coleoptera: Curculionidae) are transcontinental in distribution, occurring in sympatry in regions where their host species overlap. I tested the hypotheses that D. simplex pheromone profiles differ between mid-western U.S and southern New Brunswick populations; that overlapping pheromone components among sympatric species and eavesdropping from predators are selective pressures that may contribute to pheromone divergence in eastern D. rufipennis populations; and that host volatiles would synergize beetle pheromones, increasing attraction. Using volatile collection and gas-chromatography mass spectrometry, I found that D. simplex in southern New Brunswick produced frontalin, seudenol, and myrtenol as its main aggregation pheromone components, which were synergized by volatiles from Larix laricina (Du Roi) K. Koch (Pinales: Pinaceae). Of these, frontalin, which is also an aggregation pheromone of D. rufipennis in other parts of its range, was repellent to sympatric populations of D. rufipennis in New Brunswick. Large saltational shifts in pheromone composition reduce cross-species signaling between D. simplex and D. rufipennis and may provide an escape from predators. My findings advance our understanding of how ecological factors, including sympatry, may influence pheromone evolution, creating the potential for reproductive isolation and speciation, and have implications for the development of regionally specific lure formulations for population monitoring and pest management.