The Role of Host Plant Resistance in the Colonization Behavior and Ecology of Bark Beetles (Coleoptera: Scolytidae)

Unlike most phytophagous insects, the reproduction of primary bark beetles (Coleoptera: Scolytidae) is contingent on host mortality. Consequently, there have been intense selective pressures on trees for properties which confer resistance to attack, and likewise, on the insect for increased behavioral complexity by which to overcome these defenses. In this study, we examined the relationship between the physiology of Pinus contorta var. latifolia and the behavior of the bark beetle, Dendroctonus ponderosae. Host mortality is a discrete outcome which is contingent on the quantitative interaction between host resistance and beetle numbers. At low attack densities, trees respond by confining beetles and their associated fungi within necrotic lesions containing toxic or inhibitory compounds. Beyond a critical threshold of attack, however, the defensive capacity of the tree is exhausted, and mortality occurs. This threshold occurred at ~40 galleries/m2 in our experimental stands. The reproductive success of the bark beetle is directly related to the depletion of host defenses through concentrated attack. At sufficiently high attack densities, ~80 galleries/m2, the potential suppressive effect of the host is brood development is not manifested. Increased beetle numbers, however, cause an exponential decline in brood production due to intraspecfic competition. Consequently, there is an optimal density, ~62 attacks/m2 at which reproduction and survival of the beetle is maximized