1. Thermal performance curves (TPCs) have been estimated in multiple temperate ectotherm species and used to predict the effect of global warming. However, TPCs are typically assessed under constant temperature regimes, so their reliability for predicting thermal responses in the wild where temperature fluctuates diurnally and seasonally remains poorly documented. 2. Here we use distant latitudinal populations of five species of sepsid flies (Diptera: Sepsidae) from the temperate region (Europe, North Africa, North America) to compare estimates derived from constant TPCs with observed development rate under fluctuating temperatures in laboratory and field conditions. 3. TPCs changed across gradients in that flies originating from higher latitudes or altitudes showed accelerated development, an adaptive response. TPCs were then used to predict development rates observed under fluctuating temperatures; these predictions were relatively accurate in the laboratory but not in the field. Interestingly, the precision of TPC-predictions depended not only on the resolution of temperature data, with diurnal and overall temperature summing performing better than hourly temperature summing, but also on the frequency of temperatures falling below the estimated critical minimum temperature. Hourly temperature resolution most strongly underestimated actual development rates, because flies apparently either did not stop growing when temperatures dropped below this threshold, or they speed up their growth when the temperature rises again, thus most severely reflecting this error. 4. We conclude that when flies do not encounter cold temperatures, TPC-predictions based on constant temperatures can accurately reflect performance under fluctuating temperatures if adequately adjusted for non-linearities, but when they encounter cold temperatures this method is more error-prone. 5. Our study emphasizes the importance of the resolution of temperature data and cold temperatures in shaping thermal reaction norms, thus improving predictions of the responses of ectotherms to climate change in the age of big data and citizen science.