Phenotypic plasticity and microbial signal: unraveling the response of Hermetia illucens to long-term dietary shifts

Abstract Black soldier fly larvae (BSFL) are increasingly recognized as a promising and sustainable solution for upcycling low quality side streams into high-quality protein, but the underlying genetic and environmental mechanisms influencing their performance remain poorly understood. This study investigated the effects of cross-generational dietary changes on BSF performance and larval gut microbiome composition. We used BSF lines that had been maintained on either low-quality (wheat bran) or high-quality (chicken feed) diets for 30 generations (parental diets) and subjected them to dietary shifts across three generations (F1-F3). Results revealed improvement of growth and reproductive traits when larvae were raised on a high-quality diet, regardless of the parental diet. We also identified subtle effects of parental diet on offspring growth traits when larvae were fed the same diet. However, in most cases this latter effect was transient and disappeared in the F3 generation, suggesting a strong environmental influence on the performance traits. The gut microbiome of BSFL was significantly shaped by dietary changes, with a marked change in composition and generally higher microbial diversity when larvae were reared on wheat bran diets. In addition, a small but significant trans-generational effect of wheat bran parental diet was detected, revealing a potential signal of host/microbiome adaptive response to this diet. However, this potential link between gut microbiome composition and BSF larvae performance was not evident in the traits measured here. These findings provide valuable insight into the effects of diets on fitness components and the microbiome in BSF and offer potential avenues for optimizing farming of this species.