Effects of body size and larval diet type on reproductive traits in two diverse clades of European Lepidoptera

In insects, lifetime fecundity, egg size, and female reproductive investment are key life-history traits that – within species – strongly relate to adult body size. How these reproductive traits scale with female body size at interspecific level, and whether they are affected by larval diet type, however, is yet poorly understood. In this study, we investigated these aspects by focussing on European butterflies and Erebidae moths, two speciose and ecologically distinct groups of Lepidoptera. Species were classified, according to their primary larval diet type, as either herb feeders, woody plant/grass feeders, or alternative feeders (i.e., species associated with other diets; only for Erebidae). By accounting for phylogenetic relationships, we found maximum fecundity to scale hyperallometrically with body size, while egg size and female reproductive investment scaled hypoallometrically. These scaling relationships were consistent across both lepidopteran groups and in no case affected by larval diet type. In butterflies, however, fecundity was higher for herb feeders than for woody plant/grass feeders, while the latter were found to lay, on average, larger eggs. Diet type effects on female reproductive investment, by contrast, were only present in Erebidae, where alternative feeders unveiled an overall lower reproductive investment than herb feeders. Our study highlights the profound reproductive advantages for species to become larger (especially regarding fecundity), demonstrates that diet type relates to variation in reproductive traits among species, and asks for further research to unveil the factors driving the inconsistent diet-type effects among lepidopteran clades.