Lepidopteran species differ in susceptibility to winter warming

Predicting assemblage-level impacts of climate change requires knowledge of among-species variation in susceptibility to warming. Increased winter temperatures intuitively reduce cold stress for overwintering ectotherms, but can decrease fitness by increasing consumption of energy reserves. We kept overwintering stages of the butterflies Papilio glaucus, P. troilus (Papilionidae), and Erynnis propertius (Hesperiidae) under current temperatures and conditions simulating a 4°C increase in mean temperature, and compared mass, water, energy reserves (lipid, protein and carbohydrate content), and, in E. propertius and P. troilus, post-winter development time, adult size, and mortality. All 3 species lost mass over winter, more so during warm winters. Erynnis propertius lost more mass than either Papilio. Warm winters increased energy reserve depletion in E. propertius, but had no effect on energy reserves of either Papilio. Warming reduced development time, but did not affect mortality or adult size of P. troilus. Mortality in E. propertius was uniformly high preventing assessment of treatment effects on survival, development, or adult size. The observed overwinter mass loss by E. propertius and P. troilus was predominantly due to water loss; thus we conclude that mass loss is not an appropriate measurement of energy drain in insects, rather body composition should be measured directly. Thus, P. glaucus and P. troilus have low vulnerability to winter warming, while E. propertius shows some vulnerability. Measuring the impact of winter warming on many species will enable the identification of species traits that predict vulnerability, and facilitate identification of clades particularly susceptible to winter warming.