Doctor of Philosophy
Dr. Brent Sinclair
Winter temperatures are changing rapidly, and although winter warming reduces cold stress for overwintering ectotherms, temperature-mediated increases in metabolic rate can decrease fitness in dormant insects by increasing consumption of energy reserves. Increases in thermal variability also increase energetic demands, due to non-linear thermal response curves. My objective was to quantify the negative effects of winter warming and increases in thermal variability on a range of Lepidopteran species. As overwintering insects rely on lipid catabolism, accurate lipid measurement is central to my dissertation; so I first compared four methods of lipid quantification; concluding thin layer chromatography was the only method sufficiently accurate and robust to variation in lipid composition. I then examined the physiological and life-history costs of winter warming in Erynnis propertius [EP], Papilio glaucus [PG], P. troilus [PT], and Hyphantria cunea [HC]. A simple increase in temperature caused lipid depletion in EP, but PT, PG and HC were insensitive to winter warming. In HC, this insensitivity was mediated by a plastic suppression of metabolism and a decrease in development time in the warmer winter. HC from their northern range edge had increased thermal sensitivity at the end of winter, as predicted by metabolic cold adaptation theory. In EP, I also investigated the impact of daily thermal variability on overwintering energetics, demonstrating a facultative and obligate suppression of thermal sensitivity in response to high daily thermal variability, which partially compensated for the increased energetic demands of the more variable environment. Modelling energy use with meteorological data demonstrated that phenology changes had disproportionate influence on energetics in variable environments; thus timing of entry into winter dormancy will strongly influence ectotherm fitness in temperate environments. Metabolic suppression in EP and HC are the first demonstrations of metabolic compensation in overwintering insects. Finally, I outline a framework to predict insect vulnerability to winter warming. Winter warming and increases in thermal variability may negatively impact the fitness of some overwintering insects, but diverse physiological mechanisms compensate for increased energetic demands over winter.
Williams, Caroline M., "Overwintering energetics of Lepidoptera: the effects of winter warming and thermal variability." (2011). Electronic Thesis and Dissertation Repository. 323.