The effect of diet and temperature on life‐history traits and sexual size dimorphism in a capital‐breeding moth

In insects, temperature and diet quality during larval development are two fundamental factors affecting key life history traits that, in turn, determine an individuals' fitness. Life‐history theory predicts that within species, individuals attain larger sizes when developing under colder temperatures (i.e., temperature—size rule) or on high‐quality diet (i.e., allowing for more effective resource acquisition and development). Here, I studied how temperature and host‐plant identity affect growth rate, larval development time and size at maturity in Aglia tau L. (Lepidoptera: Saturniidae: Agliinae), a univoltine capital breeding moth. I further examined whether and to what extent these environmental variables influence the degree of sexual size dimorphism. Caterpillars were reared under two contrasting temperature regimes (constantly 18 and 23°C) and fed with three natural host plants, that is, Carpinus betulus L. (Betulaceae), Fagus sylvatica L. (Fagaceae) and Prunus padus L. (Rosaceae). A full‐factorial analysis of variance design was used to test for effects of temperature and host identity on life history traits and to account for possible interactive effects. Contrary to theory, the body sizes of males and females were maintained at higher temperatures or even increased when reared on the highest quality host plant (Prunus padus). As predicted, the high‐quality host allowed for overall larger body sizes in both sexes and further resulted in a higher degree of sexual size dimorphism. The study highlights the need to account for diet effects when studying temperature‐induced life history responses in insects and to consider further traits that may influence species‐specific reaction norms.