The effect of energetic condition on dispersal by flight and response to semiochemical cues by Dendroctonus ponderosae Hopkins (Coleoptera: Curculionidae: Scolytinae) in its expanded range in Alberta

Global climate change drives range expansion of various taxa, including the irruptive bark beetle, the mountain pine beetle (Dendroctonus ponderosae; MPB), which recently experienced a rapid range expansion. Dispersal and host colonization dictate the rate of spread and establishment of MPB populations. Mountain pine beetle undergo obligatory dispersal flights for host colonization and brood production, yet dispersal remains a poorly understood aspect of their ecology especially in the endemic population state when beetle densities on the landscape are low. Beetle morphology and energetic condition are heritable traits that explain some variation in MPB flight capacity. Selection could act on traits such as wing area, body size, and lipid reserves via spatial sorting in the expanded range of MPB. Mountain pine beetles show continuous flight polyphenisms despite similar morphology and energetic condition. A potential hypothesis to explain this variable dispersal is that energy use during flight triggers responsiveness to the semiochemicals that orchestrate host colonization of pine trees. Plasticity in host-colonization behavior is characteristic of the endemic population state, and may influence the morphology, energetic condition, and dispersal of MPB in the expanded range in Alberta. Here, I test the hypothesis that flight exercise impacts subsequent behavioral response to semiochemicals in two olfactometer experiments that simulate 1) female beetle pioneer response to host volatiles; and 2) male and female joining beetle response to host volatiles and the female-produced aggregation pheromone, trans-verbenol. I also assess electrophysiological response of antennae of female beetles to α-pinene post-flight in an electroantennogram bioassay. Finally, I use pheromone-baited funnel traps to capture MPB across its expanded range in Alberta, Canada to assess the morphology, energetic condition and dispersal of MPB in the endemic state. These experiments provide seven key findings: (1) female MPB in poor body condition due to energy expenditure respond strongly to host semiochemicals post-flight; (2) antennal response of female MPB to the host volatile, α-pinene, increases with relative lipid content of beetles; (3) male body condition or exercise by flight does not influence behavioral response to semiochemicals; (4) field traps captured 0.4% of beetles at distances indicating long-distance dispersal in the endemic population state; (5) beetles in regions along the leading edge of range expansion in Alberta have larger wings compared to beetles in the core of the expanded range; (6) the morphology and energetic condition of field captured beetles is indicative of beetles in the endemic population state; (7) MPB body lipid dynamics vary between beetles in the endemic and epidemic states. Together, my findings support the need for ongoing monitoring of endemic MPB populations that will form future outbreaks and establish a foundation for understanding of state-dependent modulation in response to semiochemical cues in MPB.

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