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

Abstract

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/m 2 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/m 2, the potential suppressive effect of the host on brood development is not manifested. Increased beetle numbers, however, cause an exponential decline in brood production due to intraspecific competition. Consequently, there is an optimal density, °62 attacks/m 2 at which reproduction and survival of the beetle is maximized. Dendroctonus ponderosae utilize defensive secretions as precursors and synergists of aggregation pheromones, and thereby, elicit maximum responses from flying beetles while the host's production of toxins is at its maximum. This mechanism also ensures that the attack will terminate once the tree has been rendered suitable for brood development and, thus, minimizes the deleterious effect of intraspecific competition. Such an interaction, in which all trees could theoretically be colonized regardless of their resistance capacity, cannot result in a stable host—parasite system. Conifers, however, seem to be able to interfere with bark beetle communication. Pheromone synthesis within the hindgut and emission from the entrance site are distinct events, and beetles contained within a copious flow of resin seem unable to elicit attraction. This ability to interrupt the beetles' communication sequence, however, is inversely related to the number of beetles initiating localized attacks. Consequently, the colonization sequence reflects the outcome of a dynamic interaction between the tree and the initial "pioneer" beetles.