Pine Wars: A New Host Interactions between the mountain pine beetle (Dendroctonus ponderosae Hopkins) and its pine hosts in Canada's boreal forest

Mountain pine beetle (MPB) has undergone a climate change facilitated range expansion and has attacked and killed trees at higher latitudes and elevations than has ever been recorded. During outbreaks, MPB attack large healthy pine trees that will fight back against the colonizing beetles using physical and chemical defenses. Attacking beetles communicate with pheromones and will cooperatively “mass attack” and kill these trees. If they win the battle against the tree, the beetles are rewarded with abundant resources under the bark but if they lose, they are poisoned or consumed by pitch. In my PhD research I studied the battle between MPB and its hosts in Alberta. Trees in the expanded range have fewer constitutive defenses and when challenged with simulated MPB attacks, are not able to produce as many toxic chemical defenses as trees in the historic range of MPB. Lodgepole pine is the most common historic host of MPB but the lodgepole pines in Alberta do not have a shared evolutionary history with MPB. I tested the hypothesis that naïve lodgepole pines are more susceptible to mass attack by performing mass attack manipulation experiments in the field. I used aggregation pheromone to attract wild MPB to experimental lodgepole pine trees and stopped the process of attack at different attack densities to determine the minimum density of beetles that can successfully colonize and kill Albertan lodgepole pines. I found variation in the threshold for mass attack across three years of experiments which was best explained by changes in environmental conditions that influenced tree defense. I then performed a similar experiment in jack pine, which is a novel host of MPB. Since there were no wild MPB populations in jack pine stands, I collected MPB from a lodgepole pine forest and transplanted them under caged jack pine trees. The mass attack threshold density in jack pine was half the beetle density typically seen in lodgepole pine which is strong evidence that jack pine is more susceptible to MPB. During the mass attack experiments, I also collected phloem tissue samples to quantify tree chemical response to MPB attack. I found that neither species increased terpene defenses in the 36 weeks after mass attack but all trees had large increases in terpene content the spring following attack, even if the mass attack was unsuccessful. Finally, I performed laboratory experiments to test the preference and performance of MPB that emerged from mass attacked lodgepole and jack pines. I found that beetles that switched hosts from lodgepole pine to jack pine had offspring that were in significantly worse condition compared to beetles that did not switch hosts or switched from jack to lodgepole pine. Although jack pine has a lower mass attack threshold and does not induce defenses quickly, there is a cost to switching from lodgepole pine to jack pine. Jack pine has a thinner phloem layer and so likely has fewer resources available for developing MPB brood. However, adaptation to jack pine could still take place.

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