BACKGROUND: Innovations to establish agricultural value chains utilising side streams and their reintegration into the feed and food supply are of great importance. Recyclates derived from biomass and waste are therefore becoming increasingly important as sources of nutrients. The larvae of the black soldier fly (BSF
Hermetia illucens) demonstrate considerable potential as livestock feed due to their ability to utilise a wide range of organic substrates. In this study, BSF larvae (BSFL) were reared on four different substrates: chicken feed diet (CD), high-fibre Gainesville fly diet (FD), or FD supplemented either with biochar (FD + BCH) or single superphosphate (FD + SSP) recyclates from sewage sludge processing. To validate the hypothesis that endogenous and substrate-associated microbiota significantly contribute to substrate conversion, the microbiota profiles of BSFL gut and frass were analysed by 16S rRNA gene amplicon sequencing. Results were associated to the different substrates as well as body composition, growth performance data, and mineral concentration of the larvae. RESULTS: The CD substrate was superior in terms of larval growth, although it caused a lower microbial alpha diversity in the larval intestine and frass compared to FD, with a dominance of Morganellaceae and families of Lactobacillales. The addition of the two sewage sludge derived products to the FD substrate significantly increased the calcium content of BSFL, while the phosphorus content was only increased by the addition of SSP. The shifts in the microbiota profiles of BSFL gut and frass indicated that BCH contributed to the regulation of the microbial milieu with suppressing the growth of potentially pathogenic microbes. The addition of SSP resulted in an enrichment of microorganisms with attributed phosphate-solubilising properties such as Pseudomonas and fungal species, likely being responsible for improving the bioavailability of phosphorus from the substrate. CONCLUSIONS: The results demonstrate the high adaptability of the BSFL and its ability to change the substrate through specific microbiota in such a way that conditions are created for an optimal nutrient supply and thus growth of the larvae.