Density-Dependent Host Acceptance Behavior as a Possible Contributing Factor to the Population Dynamics of Insect Herbivores

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9629776 Raffa Host plants can strongly affect insect population dynamics. Eruptive species undergo order-of-magnitude changes, exhibit qualitatively different phases, and impact sustainable resource management. This project will investigate interactions between population density and host acceptance behavior as a contributing factor to insect outbreaks. Specifically, population density and behavior may exert mutually positive feedback. This hypothesis will be tested using bark beetles, which are major mortality agents to conifers. Local populations are usually limited to stressed trees, but under some conditions, they can kill almost all host trees regardless of prior vigor. Underlying mechanisms by which bark beetles convert from endemic to outbreak densities are not well understood. Several aspects of their plant relations suggest a link between density-mediated host selection behavior and numerical change: 1) Beetles locate stressed trees using chemical and other cues; 2) Once entered, their likelihood of success is affected by both host resistance and the number of beetles responding to pheromones: Rapid concentration can collectively deplete host defense; 3) Field observations suggest different categories of trees may be selected during endemic and eruptive phases. To test whether changes in host selection behavior influence population dynamics, one must first understand whether individual behavior changes with density. This project will test three nonexclusive hypotheses: 1) Individuals become less discriminating as they age or deplete energy; 2) Actions of other beetles, e.g. pheromones, raise repellent thresholds; 3) Selective pressures vary with population density. These questions will be addressed using one eruptive and one non-eruptive species, the spruce beetle and pine engraver, respectively. A common laboratory assay will evaluate the effects of age, repeated rejection, flight and lipid depletion, synthetic pheromones and the presence of other beetles, and popu lation phase and response to selection. Bark beetles are responsible for severe economic damage to various conifer species. Understanding sources of variability in bark beetle host acceptance behavior will improve the ability to manage sustainable forest ecosystems.