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...