Temperature is among the most influential factors in animal biology, and especially thermal fluctuations are key determinants of life history traits for ectothermic organisms. Diet characteristics add complexity to the effect of temperature on animals' life history traits. This is even more intricate in phytophagous insects that develop inside hosts. Here, using the seed beetle Zabrotes subfasciatus Boheman we tested life history trait response to five different ambient temperatures (15, 25, 30, 35, and 45 °C). We also tested the effect of the interaction between temperature and diet on the weevils' life history traits and estimated fitness for individuals at different temperature and food supplementation conditions. We first showed that the relationship between temperature and egg size and number, and adult emergence is altered by sugar ingestion, mainly at 30 °C. Additionally, we showed that temperature and sugar ingestion have opposite effects on longevity, with this trait being heightened by sugar ingestion at 25 °C. Interestingly, except at 35 °C, egg size shows a response that opposes the temperature‐size rule, which states increasing environmental temperature reduces insect's size. Finally, excluding the extreme temperatures, fitness patterns increased with temperature and the clearer effect of parental feeding on fitness was registered at 30 °C. Since 30 °C is the temperature at which our weevil populations are maintained in the laboratory, these results point to a possible long‐term “acclimation effect” on the weevils' fitness. Thus, testing the effect of the temperature experienced by the original population on the experimental weevils' fitness might shed light on the biological explanation of the observed temperature vs diet interaction.