Developmental Parameters of a Southern Mountain Pine Beetle (Coleoptera: Curculionidae) Population Reveal Potential Source of Latitudinal Differences in Generation Time

Mountain pine beetle (Dendroctonus ponderosae Hopkins; Coleoptera: Curculionidae) is a major disturbance agent in pine (Pinus Linnaeus; Pinaceae) ecosystems of western North America. Adaptation to local climates has resulted in primarily univoltine generation time across a thermally diverse latitudinal gradient. We hypothesised that voltinism patterns have been shaped by selection for slower developmental rates in southern populations inhabiting warmer climates. To investigate traits responsible for latitudinal differences we measured lifestage-specific development of southern mountain pine beetle eggs, larvae, and pupae across a range of temperatures. Developmental rate curves were fit using maximum posterior likelihood estimation with a Bayesian prior to improve fit stability. When compared to previously published data for a northern population, optimal development of southern individuals occurred at higher temperatures, with higher development thresholds, as compared with northern individuals. Observed developmental rates of the southern and northern populations were similar across studied lifestages at 20 degrees C, and southern lifestages were generally faster at temperature extremes (10 degrees C, 27 degrees C). At 25 degrees C southern fourth instars were significantly slower than northern fourth instars. Our results suggest that evolved traits in the fourth instar and remaining unstudied lifestage, teneral (i.e., preemergent) adult, likely influence latitudinal differences in mountain pine beetle generation time.