Genetic line-specific plasticity across life stages and sexes under familiar and novel larval diets

Larval nutrition can shape phenotypes across development in holometabolous insects, but it remains unclear whether these plastic responses are shared across genetic lines and influenced by long-term dietary background. We tested larval-nutrition-induced plasticity in eight black soldier fly (Hermetia illucens) lines with divergent dietary backgrounds. Each line was reared on its familiar diet and a compositionally distinct novel diet, and we measured larval growth, pupal development, adult body weight, lifespan and sex-specific reproductive allocation. Genetic line × diet (G × E) interactions were detected across life-history traits, but were stage-specific for body mass and sex-specific for reproductive allocation. Lines differed in dietary responses for prepupal weight, whereas adult body weight showed no significant G × E interaction. Female gonadosomatic index and accessory gland-to-gonad ratio exhibited G × E interactions, whereas corresponding male traits did not. Long-term dietary background explained part of this variation. Lines generally grew faster and achieved higher pupation success on their familiar diet, whereas females reared on the novel diet showed greater ovarian investment relative to accessory glands. These findings show that larval nutrition effects differ among genetic lines and that long-term dietary background partly explains this variation, particularly in early larval growth and female reproductive allocation.