Differences in the constitutive terpene profile of lodgepole pine across a geographical range in British Columbia, and correlation with historical attack by mountain pine beetle

The mountain pine beetle, Dendroctonus ponderosae Hopkins (Coleoptera: Curculionidae), is a destructive insect pest in western Nearctic conifer forests. Currently, British Columbia, Canada, is experiencing the largest recorded outbreak of this insect, including areas that historically have had low climatic suitability for it. We analyzed 26 constitutive resin terpenes in phloem samples from British Columbia lodgepole pine (Pinus contorta) populations to test for differential resistance to mountain pine beetle attack, based upon the likelihood of previous exposure to mountain pine beetle. We assessed sampled trees for number of mountain pine beetle attacks, number of pupal chambers, and tree survival the following spring. Significant differences were found when levels of certain terpenes in lodgepole pine populations that had likely experienced substantial mountain pine beetle infestations in the past were compared with those in populations that likely had not experienced large outbreaks of mountain pine beetle. Although we expected southern pine populations to contain more total terpenes than northern populations, owing to higher historical exposure to the beetle, the converse was found. Northern populations generally had higher levels of constitutive terpenes and beetle attack than southern populations. Because several terpenes are kairomones to the mountain pine beetle and also serve as precursors for the synthesis of pheromones, the lower levels of terpenes expressed by lodgepole pines from the historical range of the mountain pine beetle may render them less chemically perceptible to foraging beetles.