Genetic variation of lodgepole pine (Pinus contorta var. Latifolia [Engelm.]) chemical and physical defenses that affect mountain pine beetle Dendroctonus ponderosae attack and tree mortality

The current outbreak of mountain pine beetle (MPB), Dendroctonus ponderosae (Coleoptera: Curculionidae), and its fungal complex Grosmannia clavigera has affected ∼15 million hectares of lodgepole pine Pinus contorta var. latifolia (Engelm.) in British Columbia. This is of great concern to forestry industry in British Columbia but it has provided a rare opportunity to study genetic variation of lodgepole pine defense mechanisms against MPB. Based on a previous study, we selected 45 open-pollinated families from a population of 180 families, with the number of trees per family ranging from 16-26 (for a total of 887 trees) from two 20-year field tests which had undergone attack from MPB. Chemical and physical defensive responses were quantified in response to simulated attack by the MPB fungal complex on these 45 families. These responses included: physical exudation of resin, host compartmentalization response to fungal inoculation, and levels of constitutive and induced resin terpenoids. Further assessments were made for tree height, DBH (diameter at breast height ∼1.4 m), bark texture, tree mortality, and MPB presence, attack density, reproduction, and brood development, as well as the presence of hypersensitivity reactions by the tree to MPB. Data suggest that variation in resin terpenoids pre- and post-induction of simulated MPB attack is significantly different between families. Traits in lodgepole pine that affect MPB colonization and tree mortality also vary among families, and many host tree traits that affect the MPB lifecycle are significantly heritable. In order to examine genetic interrelationships among traits, family mean correlations were calculated for traits with significant heritabilities (as an approximation of the underlying genetic correlation). Constitutive (pre-induction) delta-3-carene was negatively correlated with family mean mortality. The frequency of hypersensitivity reactions was negatively correlated with MPB gallery production, reproduction, and brood development. delta-3-Carene and hypersensitivity reaction frequency negatively impact MPB induced tree mortality and likely subsequent MPB fitness. These results provide important future directions for studying genetic resistance in lodgepole pine to bark beetles, as well as forest managers trying to manage future MPB outbreaks in lodgepole pine.