Dietary shifts illuminate host-microbiome-diet interplay in black soldier fly larvae.

Understanding the tripartite interaction between diet, the microbiome, and host physiology is essential for optimizing the bioconversion efficiency of black soldier fly larvae (Hermetia illucens). These interactions were investigated by analyzing life-history traits and metatranscriptomes of larvae fed carbohydrate-rich (1P:5C), protein-rich (5P:1C), and balanced ratio macronutrient (1P:1C) diets. The results showed that dietary macronutrients correlated with shifts in the microbial community and gene expression. In particular, optimal larval performance, characterized by the highest weight and survivorship, was achieved on the balanced ratio diet. While the carbohydrate-rich diet increased microbial alpha diversity and enriched microbial transcripts for carbohydrate metabolism and transport, it significantly delayed pupation, reduced larval weight, and induced host immune genes related to pathogen recognition. Larval guts were predominantly composed of the fungal genus Magnusiomyces, no matter the diet, whereas the frass microbiome was primarily bacterial. In the frass, Klebsiella dominated the carbohydrate-rich diet, while Streptococcus, Lactobacillus, and Klebsiella were the most prevalent taxa in the balanced and protein-rich treatments. Significant correlations were identified between host gene expression and microbial alpha diversity and transcript expression, suggesting host-microbe crosstalk in response to nutritional stress. Ultimately, these findings demonstrate that balanced macronutrient ratios are required to synchronize host-microbiota metabolic synergy and mitigate physiological stress, providing a definitive mechanistic framework for optimizing industrial black soldier fly rearing outcomes.