The body size of insects is affected by environmental conditions during development and can present considerable intraspecific variations, which can be seen as an ultimate consequence/adaptation to environmental conditions. This paper evaluated whether the body size of the butterfly Dryas iulia from subtropical populations was influenced by changing climate conditions and food source availability during the seasons. The likely reasons behind body size variation were also investigated. First, field data on body size variation, host-plant availability and climate fluctuation throughout the seasons were recorded. Then, the effects of host-plant species and temperature on body size were analyzed by controlled experiments. Field data revealed that body size and host-plant availability varied significantly through the seasons. Populations had the smallest body size during the spring and the biggest size during summer, whereas host-plant availability was lower during winter and higher during spring. The controlled experiments revealed that both temperature and host-plant had significant effect on the plasticity of body size. Larvae subjected to winter temperature treatment led to smaller butterflies when compared to immatures reared under summer temperature treatment, and larvae fed with Passiflora misera produced bigger adults when compared to larvae reared on Passiflora suberosa. The combination of data gathered in the field and in the laboratory suggests that seasonal body size variation in D. iulia is related mainly to differences in the temperatures to which larvae are subjected during development, while host-plant shifts caused by differential availability of food through the seasons had slightly effects on the variation observed.