Adverse environmental conditions constrain active flight and thereby limit reproduction in most insects. Butterflies have evolved various adaptations in order to thermoregulate, allowing females to search for nectar and to oviposit under unfavorable thermal conditions. We studied experimentally and with observational data the effect of low ambient temperatures experienced in the morning on the timing of oviposition and clutch size in the Glanville fritillary butterfly (Melitaea cinxia). Comparisons were made between individuals with different forms of the gene Pgi, encoding the glycolytic enzyme phosphoglucose isomerase, since naturally segregating variation at Pgi is known to be correlated with flight metabolic rate, flight performance, and fecundity. Experiencing low temperature in the morning delayed the initiation of oviposition and decreased clutch size. We used a thermal image camera to measure the thoracic surface temperature of butterflies immediately after voluntary flight. Single nucleotide polymorphism at Pgi was associated with thoracic temperature at low ambient temperatures. This has consequences for reproduction because females that are able to fly at lower ambient temperatures generally initiate oviposition earlier in the afternoon, when the environmental conditions are most favorable and the average egg clutch size is generally largest. These results suggest that variation in physiological and molecular capacity to sustain active flight at low ambient temperature has significant fitness-related consequences in insects.