Spatiotemporal evolutionary dynamics and biological response of mountain pine beetle (Dendroctonus ponderosae Hopkins) (Coleoptera: Curculionidae) in western Canada after range expansion

Global ecosystems are increasingly affected by climate change leading to alterations in expected disturbance regimes. Outbreaks of irruptive insect pests represent some of the most destructive disturbance events possible that occur in the forests of North America. The mountain pine beetle, Dendroctonus ponderosae Hopkins (Coleoptera: Curculionidae) (MPB), an irruptive tree-killing pest of several species of pine, is responsible for the most damaging insect outbreak in recent history in western Canada, leading to a massive range expansion from established territory in British Columbia to pine stands in central and northern Alberta during the mid-2000s. The adaptive genetic structure of these new populations is relatively unknown a decade after range expansion and the selective response of the beetle to novel habitats is likewise understudied. There is increasing concern among forestry stakeholders that MPB may develop novel genetic traits or behaviours in response to their new habitat. In this study, I sampled beetle DNA and used double-digest restriction fragment genotyping-by-sequencing to generate single nucleotide polymorphism (SNP) loci. I used these data to investigate the establishment of genetic structure throughout the newly expanded and historic MPB range. I identified two distinct genetic clusters, a southern cluster for beetles located south of Banff National Park and a northern for beetles located north of the park. The data presented here suggest that Jasper National Park and the surrounding region represents an area of admixture between the two genetic clusters, caused in part by the movement of beetles from both the north and the south. This area of admixture may have the potential to differentiate into a separate third genetic cluster. Outlier analysis indicated that several landscape variables including mean annual precipitation and relative humidity contributed to the selective pressures on MPB, while frost free period contributes to the genotypes of beetles from the expanded range in central and northern Alberta. We also found that novel colonized MPB sites caused by in-flights from BC to Alberta did not display changes in genetic structure from their source population. Semivoltine MPB new adults collected from a site within the admixed zone near Jasper National Park displayed accumulation of cryoprotectants in response to cooling autumn temperatures but are not capable of surviving below -30℃. This is the first time that new adults have been demonstrated to cold harden, but also indicates that beetles from the admixed zone are likely not displaying aberrant forms of dispersal, host selection, or cold tolerance. I propose that beetles within the expanded range are not experiencing extreme selection events that would lead to genetic changes that will cause notable colonization, gustation, or dispersal differences to that of their counterparts in the historical range of BC. Warming climate is likely to contribute to beetle survival throughout central and northern Alberta, however, MBP populations will likely reach an endemic state and display historically expected population cycling (endemic to epidemic to endemic) in the future. The homogenous nature of MPB genetic structure throughout the expanded range does not support the development of populations-specific control techniques.

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