Bark beetle

Abstract: Climatic changes are predicted to significantly affect the frequency and severity of disturbances that shape forest ecosystems. We provide a synthesis of climate change effects on native bark beetles, important mortality agents of conifers in western North America. Because of differences in temperature-dependent life-history strategies, including cold-induced mortality and developmental timing, responses to warming will differ among and within bark beetle species. The success of bark beetle populations will also be influenced indirectly by the effects of climate on community associates and host-tree vigor, although little information is available to quantify these relationships. We used available population models and climate forecasts to explore the responses of two eruptive bark beetle species. Based on projected warming, increases in thermal regimes conducive to population success are predicted for Dendroctonus rufipennis (Kirby) and Dendroctonus ponderosae Hopkins, although there is considerable spatial and temporal variability. These predictions from population models suggest a movement of temperature suitability to higher latitudes and elevations and identify regions with a high potential for bark beetle outbreaks and associated tree mortality in the coming century.

Abstract: Unlike most phytophagous insects, the reproduction of primary bark beetles (Coleop- tera: 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 on brood development is not manifested. Increased beetle numbers, however, cause an exponential decline in brood production due to intraspecific competition. Conse- quently, there is an optimal density, =62 attacks/M2 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 in- versely 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.