Thermal performance under constant temperatures can accurately predict insect development times across naturally variable microclimates

External conditions can drive biological rates in ectotherms by directly influencing body temperatures. While estimating the temperature dependence of performance-traits such as growth and development rate is feasible under controlled laboratory settings, predictions in nature are difficult. One major challenge lies in translating performance under constant conditions to fluctuating environments. Using the butterfly Pieris napi as model system, we show that development rate, an important fitness trait, can be accurately predicted in the field using models parameterized under constant laboratory temperatures. Additionally, using a factorial design, we show that accurate predictions can be made across microhabitats, but critically hinge on adequate consideration of nonlinearity in reaction norms, spatial heterogeneity in microclimate, and temporal variation in temperature. Our empirical results are also supported by a comparison of published and simulated data. Conclusively, our combined results suggest that, discounting direct effects of temperature, insect development rates are generally unaffected by thermal fluctuations.
Methods
Thermal performance in development rate was measured at 8 constant temperatures in the butterfly Pieris napi. Measurements were made for eggs and larvae separately, as well as for the full ontogonetic development between oviposition and pupation (eggs and larvae combined). Thermal performance curves were fit to the data. Prediction models were parameterized based on this data, and validated through field transplants. For the field transplants, microclimate temperatures were frequently sampled at multiple sites. These temperatures were used to predict development times. For comparison, weather station data was also used in the prediction model. Transplanted individuals were monitored and their development times in the field were compared to predictions.

All raw data necessary to reproduce these results are available here, and compressed to "von_Schmalensee_et_al_2021_ecol_lett_scripts_and_data.rar". Additionally, the scripts used to produce the results and the five main figures are available, with annotation. See the "0_readme.txt" file for more information, and the main manuscript and supporting information for a detailed description of the methods.