Interaction Dynamics of Bark Beetle Aggregation and Conifer Defense Rates

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Abstract
The requirements of host tree death for successful reproduction by some bark beetles (Coleoptera: Scolytidae) has placed strong selective pressures on conifers for rapidly induced, complex defensive systems. Conversely, the ability of bark beetles to engage in cooperative attacks mediated by aggregation pheromones and their associations with phytopathogenic fungi enable them to overcome these defenses under certain conditions. Although the outcome of colonization attempts is discrete, tree defensive capacities vary quantitatively within a population. There are multiple interactions among these features, and so the separate processes that govern conifer-bark beetle systems have proven difficult to study or manipulate for improved forest protection. A model is developed which delineates the key parameters of colonization success or failure. The differential equations that structure the model are based entirely on validated assumptions from previous field and laboratory work. Using this model, various assumptions regarding the effects of such factors as environmental stress, underlying resistance, and beetle immigration can be evaluated for each conifer-scolytid system. This model may also facilitate the development of comparative approaches for addressing coevolutionary interactions among bark beetles and their host plants.