Multitrophic constraints on the reproductive success of eruptive bark beetles: Interactions of population density and chemical signaling with host trees, natural enemies and microorganisms

Dendroctonus ponderosae intermittently escapes biotic and abiotic constraints, reaching eruptive population densities and killing healthy trees. Constitutive and induced physiological characteristics were measured in lodgepole pine stands with varying D. ponderosae densities. At low densities, unattacked trees had higher constitutive and induced resin flow and induced total monoterpenes than attacked trees. These relationships were lost at intermediate densities and reversed at high densities. Beetles attacked progressively larger trees, which had higher defenses, as their populations increased. Thus, tree resistance can be a critical mediator of bark beetle population dynamics at endemic levels, and inconsequential after a critical population density is surpassed.;A secondary bark beetle, Ips pini, can both compete with and attract additional predators to D. ponderosae. Temnochila chlorodia was attracted to D. ponderosae pheromones. Enoclerus lecontei was attracted to I. pini pheromones. Combinations of I. pini and these predators were sequentially added to logs colonized by D. ponderosae. The combination of I. pini, T. chlorodia, and E. lecontei reduced D. ponderosae's reproduction below that with I. pini alone.;Microbial symbionts of bark beetles may provide volatile cues for natural enemies seeking cryptic prey. The specialist parasitoids, Heydenia unica and Rhopalicus pulchripennis were more attracted to logs colonized by I. pini than logs alone and blank controls. Heydenia unica was equally attracted to logs containing I. pini or logs inoculated with O. ips. The generalist parasitoid, Dibrachys cavus was more attracted to treatments containing logs than blank controls. Two dipteran predators, Medetera sp. and an unidentified dolichopodid, were more attracted to logs than blank controls, but their responses were less consistent and pronounced than those of specialist parasitoids.;Colonization by bark beetle-microbial complexes creates a habitat for diverse assemblages of insects. Parasitic hymenoptera collected on blank controls, host plants, host plants with microbial symbionts, and host plants with I. pini, included Braconidae, Platygastridae, Encyrtidae, Pteromalidae, and Ichneumonidae. Seven bark beetle parasitoids were captured. Parasitoids of Diptera, Lepidoptera, Hymenoptera, and non-wood-boring Coleoptera were also common. Nineteen parasitoids of phytophages, fungivores, and saprophytes showed preferential attraction to some of the above treatments over controls.