Dietary protein-to-carbohydrate ratio affects larval growth and antibacterial immune response in the black soldier fly

Black soldier fly larvae (BSFL) have emerged as a promising and sustainable source of ingredients for animal feed due to their rapid growth and ability to efficiently bioconvert different organic waste and by-products into high-nutritional-quality insect biomass. Although considerable research has focused on the optimization of larval rearing conditions to improve the chemical composition of the insect biomass, the interplay between diet composition and immune function in BSFL remains poorly understood. Herein, we investigated how two diets differing in their protein-to-carbohydrate (P:C) ratio affect larval performance and the immune response following bacterial challenge. Our results demonstrate that dietary P:C ratio influences both growth and immunity in BSFL. Specifically, larvae fed on the diet with lower P:C ratio (0.32) reached a higher body weight than those grown on the higher P:C ratio diet (1.28), while developmental time and adult emergence rate were unaffected. In contrast, BSFL reared on the higher P:C ratio diet exhibited an enhanced immune response, characterised by an increased number of circulating haemocytes and upregulated expression of antimicrobial peptides genes, resulting in a more rapid bacterial clearance. Collectively, our findings demonstrate that, under these two dietary conditions, macronutrient composition drives distinct trade-offs between growth and antibacterial immune response in BSFL. These results provide a foundation for future studies aimed at identifying nutritionally optimised feeding strategies that balance biomass production with disease resistance under industrial rearing conditions.