Density related variability in host selection by bark beetles: Feedback between individual and population level processes

This research examined whether host selection behaviors of individual insects change with and contribute to the population dynamics of eruptive herbivores. We examined one non-eruptive and one eruptive species of bark beetles. Bioassays indicate that beetles of both species are repelled by high concentrations of the predominant monoterpenes in their hosts, but intermediate concentrations elicit entry and gallery construction behaviors. Each beetle's decision to enter phloem-based media amended with host phytochemicals was influenced by the types and concentrations of monoterpenes present. Previous encounters with a simulated host containing a moderate concentration of monoterpenes increased the likelihood that the same concentration would be accepted upon subsequent encounters. Effects of alpha-pinene and limonene on host entry behavior were mediated by the density of beetles on the surface of the assay arena, and by the density of beetles constructing galleries within the medium. Entrance rate increased until a threshold density of 3 to 4 beetles per assay arena. Gallery lengths increased in the presence of aggregation pheromones and density of beetles within the assay arena. Similar responses were observed with D. rufipennis, but they varied with population phase. Heritability of host acceptance and gallery construction behaviors were estimated at 0.76 and 0.6, respectively.; We conducted field and laboratory experiments on D. rufipennis from 14 endemic and 15 eruptive populations in Alaska and Utah, USA, and Yukon, Canada. Felled trees were colonized by beetles in both population phases, and live trees were colonized by beetles in the eruptive phase. The adult progeny of beetles that colonized live trees were less deterred by monoterpenes than beetles from felled trees in the same sites. Beetles from eruptive populations were less likely to enter medium and constructed shorter galleries. Reduction in aversion to high monoterpene concentrations elicited by the presence of other beetles is more pronounced among beetles from eruptive than endemic populations. We propose that the above modifications of behaviors may contribute to positive feedback in population dynamics and discuss linkage between host breadth and increased population densities in eruptive species.