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The requirement 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 coniferbark 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 coniferscolytid system. This model may also facilitate the development of comparative approaches for addressing coevolutionary interactions among bark beetles and their host plants.

Summary — The monoterpene response of phloem and sapwood of individual pines belonging to 3 species (Pinus contorta, Pinus ponderosa and Pinus monticola) to inoculation with Ophiostoma clavigerum and injection with chitosan, a proteinase inhibitor-inducing factor and a control buffer, was investigated quantitatively and qualitatively. The total quantity of monoterpene in the reactive tissues increased with each treatment but to different levels. In each tree, the monoterpene composition of the reactive tissues differed from that of the unwounded tissues, but was the same whatever the treatment, even in the case of an injection with buffer control. In addition, phloem and sapwood responses were qualitatively identical although constitutive compositions differed greatly. The composition of reactive tissues was not very different from that of unwounded sapwood. The direction of variation of each monoterpene from unwounded to reactive tissues differed according to the particular tree. Only phellandrene + limonene reacted consistently. From these results we cannot conclude that chitosan is a natural elicitor, and the non-specificity of the response for the aggression favors the hypothesis that an elicitor originates from the tree itself. Because of this non-specificity, and the fact that the three trees responded in a qualitatively different manner, we suggest that the qualitative monoterpene response of the tree is not adapted to any specific aggressor even though these trees are usually hosts of the same bark beetle-fungus complex. Thus, the role of monoterpenes in the induced defensive response is very likely a quantitative and dose-dependent relationship.

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