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.

Income breeding animals support reproduction by the intake of energy and molecular building blocks during adulthood. Capital breeders, in contrast, fuel reproduction by the intake of materials during the larval stage. There exists, however, some controversy as to whether adult feeding in capital breeders actually increases fitness. We tested whether individuals feed as adults in the weevil Zabrotes subfasciatus Boheman, whether they equally accept carbohydrate‐ or nitrogen‐rich food (sugar or yeast), whether food ingestion varies with the available host type and whether feeding as adults alters life history traits. The hosts tested were three varieties of Phaseolus vulgaris, Carioca, Jalo, and Preto; Vigna unguiculata Frade; and Vicia faba. We also tested whether sugar ingestion affects life history traits differently in the presence of different host types. We found that Z. subfasciatus feed as adults and equally accept sugar and yeast, and food ingestion depends on host type. Sugar and yeast significantly promote oviposition of larger eggs on Carioca seeds and enhance adult emergence, and sugar ingestion increases longevity. Moreover, the insects avoid feeding when on V. faba seeds, on which they did not oviposit. Mean fitness values showed the following hierarchy: Carioca = Jalo > Frade > Preto > V. faba. These results show that the capital breeder Z. subfasciatus feed as adults, which increases life history traits and suggests the existence of a hierarchy of performance on different hosts, with the P. vulgaris varieties Carioca and Jalo offering the best conditions.