Dietary carbohydrate structure affects the gut microbial diversity, succession, and compartmentalization in black soldier fly larvae

Introduction Black soldier fly larvae (BSFL) plays a pivotal role in bioconverting organic wastes into high-value biomass by gut microbiota. However, how dietary carbohydrate structure modulates gut microbial assembly remains unclear. Methods Here, we characterized the gut bacterial communities across developmental stages and gut regions of BSFL reared on wheat bran (fiber-rich) and cassava (starch-rich) diets using 16S rRNA sequencing. Alpha-diversity analysis revealed that wheat bran diet significantly enhanced species richness (ACE, Chao) and diversity (Shannon), with progressive increases across larval stages, whereas cassava diet only increased richness without improved evenness. Dietary substrate was the primary driver of gut microbiota composition (PC1: 32.5% variance), followed by developmental stage. Proteobacteria, Firmicutes, Bacteroidota, and Actinobacteriota were the predominant bacterial phyla across both diets. Firmicutes dominated the wheat bran group (52.34%) with a successional shift from Proteobacteria-enriched early stages to Firmicutes-enriched later stages, while Proteobacteria prevailed in cassava group (79.27%) throughout development. The wheat bran group was dominated by the genera of Klebsiella (20.17%), Bacillus (18.17%), Providencia (10.15%), whereas cassava group by unclassified_f_Enterobacteriaceae , Klebsiella , Enterococcus , Providencia , and Morganella . Specially, Bacillaceae prevailed in the anterior gut and the hindgut and Lachnospiraceae in the hindgut under wheat bran diet, indicating sequential fiber degradation. Cassava diet exhibited a homogeneous Enterobacteriaceae dominance across all regions. LEfSe further identified Firmicutes-affiliated taxa enriched in wheat bran and Dysgonomonas-associated taxa in cassava. Discussion Collectively, these findings demonstrate that dietary carbohydrate structure dictates the taxonomic composition of the BSFL gut microbiota, offering practical guidance for optimizing waste bioconversion stratˇegies based on feedstock characteristics.