Nutrient–Microbiota Co-Regulation of Protein Conversion in Black Soldier Fly Larvae: The Role of Alkali-Soluble Protein and Gut Microbial Communities

Insect protein farming offers sustainable advantages in land efficiency, emission reductions, and bioconversion, yet optimizing the nutrient composition remains a major challenge for cost-effective production. This study investigates the co-regulatory mechanism between alkali-soluble protein (SpA) and the gut microbiota in black soldier fly larvae (Hermetia illucens) and their effect on protein conversion efficiency. Feeding trials with varying alfalfa/SpA ratios identified a wheat middlings/alfalfa meal blend at a (5:0 ratio) as optimal for promoting larval protein accumulation. SDS-PAGE and 16S rRNA analyses revealed a strong positive correlation between SpA and larval crude protein (R2 = 0.82). The network analysis and Pearson correlation heatmap further confirmed positive correlations among SpA, larval protein, Enterococcus, and Ignatzschineria (p < 0.05), suggesting that high SpA in the substrate was associated with the enrichment of these taxa, which synergistically enhanced proteolysis through alkaline protease secretion (R2 = 0.85) and chitinase-mediated gut remodeling. Multi-linear regression modeling verified SpA as a superior predictor of the crude protein content compared with total nitrogen (TN), improving the model’s coefficient of determination (R2) from 0.40 to 0.82. These findings highlight SpA’s higher bioavailability and its direct role in metabolic utilization. By integrating the feed composition, microbiome function, and host metabolism, this study established a regulatory network driving larval protein biosynthesis. The targeted modulation of dietary SpA content may offer a promising approach to enhance beneficial microbial communities and improve protein conversion efficiency in BSFL-rearing systems. These findings provide a theoretical basis for optimizing feed formulations to support sustainable insect protein production from organic waste.