Diet chemistry and cross-kingdom microbiota associate with black soldier fly larvae performance on regional side streams

Black soldier fly larvae (BSFL; Hermetia illucens) are increasingly farmed as a sustainable source of animal protein and are capable of converting diverse organic material into high-value biomass. Agricultural side streams represent an abundant and underutilized feed resource within the European Union (EU). However, their influence on BSFL development and gut microbial communities remains insufficiently characterized despite the central role of the microbiota in host nutrition, immunity, and health. We evaluated five feed treatments, namely chicken feed (control), apple pomace, potato pulp (industrial-scale and lab-scale), and rapeseed cake, for their suitability in supporting BSFL growth from hatching to pupation. All feed treatments supported larval development, with time to pupation ranging from 25 days (rapeseed cake) to 87 days (potato pulp industrial-scale). Larvae reared on chicken feed exhibited significantly (7.6-fold) higher biomass compared to those fed with apple pomace, while the performance of BSFL reared on rapeseed cake was similar to that of the control larvae. Variations in development performance correlated with differences in feed nutrient profiles, particularly protein, fat, and fiber content. Furthermore, gut microbiota profiling via 16S rRNA and ITS2 amplicon sequencing revealed that gut microbial communities varied depending on diet chemistry. This was supported by functional predictions indicating an enrichment of fiber-degrading and nitrogen-fixing bacteria in larvae fed fiber-rich, nitrogen-poor diets. Notably, no methane production of L5 larvae was detected across treatments under tested laboratory conditions. This study demonstrates the potential of agricultural side streams to serve as viable feed substrates for BSFL production, with associated shifts in gut microbiota likely supporting host adjustment to less-optimal diets. These findings underline the importance of microbiota plasticity in BSFL digestion and highlight the relevance of feed composition in optimizing large-scale insect farming for sustainable protein production.