VARIABILITY IN THERMAL SENSITIVITY OF ADULT REPRODUCTIVE DIAPAUSE IN THE SPRUCE BEETLE, DENDROCTONUS RUFIPENNIS

Abstract

Diapause induction and termination, which regulate seasonal insect life cycles, can be highly variable within and among populations because these traits are often genetically variable and sensitive to environmental fluctuations. Both types of variation may influence how insect populations respond plastically or evolutionarily to changing climates. In this study we assessed variability in the expression of reproductive diapause in spruce beetles (Dendroctonus rufipennis), a major forest pest whose life cycle timing is regulated by both a prepupal and adult reproductive diapause. Prepupal diapause is facultative and dependent on temperature in the field, whereas previous studies suggest that adult reproductive diapause is effectively obligate. We tested for variability in adult reproductive diapause termination within and between two populations of spruce bark beetle collected from sites in Colorado and Wyoming and reared under warm, summer-like conditions in the laboratory. We also sampled beetles from under tree bark during the fall and spring to estimate how reproductive diapause might terminate naturally in the field. We present evidence that though most beetles induce and do not terminate diapause under constant warm conditions, a small proportion of females from both populations developed mature ovaries and successfully reproduced under warm conditions in the lab. Previous studies have suggested that most beetles require exposure to relatively low temperatures for several weeks to months to terminate diapause in the lab. We found that most female beetles sampled in the field had mature ovaries relatively early in the fall, suggesting that exposure to transiently low temperatures in the field may rapidly terminate adult reproductive diapause. Thus, adult reproductive arrest may primarily act as a block to prevent offspring production prior to winter and appears unnecessary for survival overwinter. Overall, our data do not suggest that major shifts in spruce beetle life cycles as mediated by adult reproductive diapause are immediately imminent with changing climates but, if the variability that we observed is heritable, adult reproductive diapause may have some capacity to evolve in both populations. ** Note ** all figures/tables designated with S are supplemental figures/tables that will appear only in the supplement in the published version. They are provided here alongside the main figures/tables, in-line in the order that they appear in the text for convenience.