Butterflies (Lepidoptera) can absorb heat from sunlight to increase their thoracic temperature above the threshold for flight. Autonomous flight is closely related to the reproductive success of butterflies; however, it is unclear whether light intensity controls the ability of butterflies to absorb heat and alter flight activity and, thus, affect reproduction. Such insight would provide an improved understanding of the utilization of solar heat by insects. In this study, we investigated the flight behavior of Danaus chrysippus (L.) (Lepidoptera: Danaidae) in the field and measured the thoracic temperature of butterflies when flying and perching. We determined the effects of thoracic temperature on flight frequency and reproductive success under a light intensity range of 2,000–45,000 luminous flux per unit area (lx). Within this range, strong sunlight significantly increased the flight activity of butterflies in the field. Adults absorbed heat from sunlight to maintain a thoracic temperature that was 4–4.5°C higher than the ambient temperature to allow autonomous flight. Light intensity between 15,000 and 45,000 lx enabled butterflies to absorb more heat to reach an equilibrium temperature, increasing the frequency of autonomous flight and mating. In summary, light intensity influences the autonomic flight activity of butterflies and then significantly affects mating frequency and reproduction. Thus, light intensity manipulation can be used to regulate butterfly reproduction for their conservation and utilization in laboratory breeding facilities.