Investment of resources in reproduction can be based on individual state, environmental conditions, and perceived mate quality. Changing climates impact many aspects of the environment by increasing temperature, decreasing precipitation, and altering resource availability. Access to high-quality resources is known to improve survival under elevated temperatures, but its effects on reproduction in warming environments are largely unexplored. Here, we investigate the effects of elevated temperature and sugar resources on reproductive output within and between E- and Z-pheromone strains of the European corn borer moth [Ostrinia nubilalis (Hübner) (Lepidoptera: Crambidae)]. Corn borers prefer mates from their own strain, with reproductive output being highest for within-strain pairs. In this experiment, mating pairs were provided with a 20% sugar solution while exposed to either ambient (23 °C) or elevated (28 °C) temperatures. We measured reproductive investment as the total number of egg clusters laid 3 days after pairing. We found that at ambient temperature, sugar supplementation resulted in high investment across all pairs, including with usually unpreferred mates. However, when sugar was provided at elevated temperature, more egg clusters were laid in pairs with preferred (within-strain) mates as compared to less preferred (between-strain) mates. These results differ from temperature effects in the absence of sugar and suggest that the effects of sugar on reproductive investment in less preferred mates depend on temperature. Changes in investment may be due to differences in the allocation of extra resources to thermoregulation at elevated temperatures. Our results suggest the possibility of interactive effects of temperature and resources on sexual selection.