Extreme events, such as heatwaves can have detrimental effects on plant and animal populations. The frequency, intensity and duration of heatwaves are predicted to increase as Earth’s mean temperature keeps rising. Climate change has been identified as a contributing factor to recent declines of insect populations; however, the specific mechanisms that lead to reduced population sizes still remain relatively understudied. Here, we assessed the responses of a North American butterfly, the silver-spotted skipper (Epargyreus clarus), to heatwaves at different ontogenetic stages (eggs, early, and late instar larvae). Using growth chambers in the laboratory, we subjected individuals to a typical summer temperature regime in the southeastern range of 26 ± 5◦C (mean ± amplitude), which was interrupted by a 3-day heatwave of either low (31 ± 10◦C) or high (36 ± 5◦C) intensity. Note that the maximum temperature in both heatwave treatments was 41◦C. We used linear and generalized linear models to compare survival, development time and pupal mass among treatments.
Results/Conclusions
Heatwave intensity had a large effect on egg survival, as 82% eggs (N = 39) exposed to the low intensity heatwave survived compared to only 2% of those exposed to the high intensity heatwave (N = 43). By contrast, survival of larvae under thermal stress was independent of heatwave intensity (χ2 = 1.33, P = 0.24, N = 173). Ontogenetic stage was an important predictor of larval survival, as late-instar larvae were more likely to survive under thermal stress (95% survival) than early-instar larvae (75% survival, χ2 = 4.55, P = 0.03). In addition, experiencing a heatwave during the late instars was associated with a pupal mass reduction of 4% compared to individuals that experienced the heatwave during the early instars, F1,137 = 5.6, P = 0.02, N = 141). Overall, we found that the magnitude of the negative effects of heatwaves on E. clarus changes across ontogeny and that eggs were the most susceptible immature life stage. While it is commonly accepted that maximum thermal physiological limits are well conserved within taxa, it is important to consider differences in thermal limits among ontogenetic stages when assessing the effects of thermal stress.