Biological Barriers to Forest Pest Invasions: A Novel Host Tree Slows Mountain Pine Beetle Range Expansion.

Mountain pine beetle breached the Canadian Rocky Mountains-a former geographic barrier-initiating an eastward range expansion that threatens pine forests across North America. However, mountain pine beetle's expansion stalled unexpectedly in eastern Alberta, defying predictions of rapid spread through jack pine, a novel host tree. We investigated the mechanisms behind this slowed spread using an integrated methodology combining helicopter survey data, statistical modeling, simulations, and a consideration of experimental data. While previous hypotheses attributed the slowed spread to lower pine volumes and stem densities in eastern Alberta's forests, our findings indicate that jack pine's inherent phenotypic characteristics-specifically its smaller size, thinner phloem, and lower monoterpene concentrations-are the main factors limiting beetle success. Mountain pine beetle's limited spread is primarily caused by difficulties in locating and successfully attacking jack pine trees, rather than challenges with reproduction or larval survival within jack pine. Jack pine's traits appear to provide natural resistance against mountain pine beetle invasion, suggesting a lower risk of continued eastward spread than previously assumed. However, given the significant implications for forest management policy and the uncertainties inherent in ecological forecasting, we recommend maintaining beetle monitoring programs.