Identification of Optimal Attack Density of MPB in Hybrid and Jack Pine in Its Expanded Range in Alberta

Mountain pine beetle population dynamics depend on population density. At low densities, in the endemic population phase, beetles persist by preferentially using and reproducing in weak trees. At high population densities, in the epidemic population phase, beetles preferentially attack stronger trees by mass attack allowing them to colonize and kill large, healthy trees. The threshold density of attacking beetles required for mass attack of healthy trees, is ~40 attacking beetles per square metre.

Our initial work, as part of the TRIA-Net project, has shown that the optimal attack density is shifted lower in naïve lodgepole pine trees in the expanded range in Alberta. This implies that fewer beetles would be needed to establish a mass attack to kill healthy lodgepole pine trees in the expanded range. However, it is not clear whether this also translates into a lower mass attack threshold for mountain pine beetle attacking naïve hosts of different species including jack pine and lodgepole x jack pine hybrids. As part of the TRIA-Net project we also set out to determine the threshold density required for successful mass attack by mountain pine beetle in these novel host species. Although we established an innovative method to induce mass attacks in hybrid and jack pine stands with low beetle activity, we were unsuccessful in inducing sufficient attack density using conventional mountain pine beetle pheromone baits in these stands.

Because quantification of the optimal attack density in jack pine and hybrid trees is critical for understanding mountain pine beetle dynamics in novel habitats, we feel it is crucial to continue our work in these stands beyond the end of the TRIA-Net project (June 2018). We will adjust our methods to use the modified pheromone bait developed by the Erbilgin lab in addition to release of beetles in areas of low density to ensure sufficient attack densities to answer our question.