Agro-industrial by-products for black soldier fly larvae: gut microbiota, performance and probiotic application

Larvae of black soldier fly (Hermetia illucens L.) efficiently bioconvert low value organic mass into high quality biomass enabling the valorisation of agri-food side-streams within circular bioeconomy paradigms. Scaling up black soldier fly production in a sustainable and cost efficient manner, requires the identification of feedstocks economically accessible and available at industrial volumes. Diet composition is known to modulate not also larval performance, but also gut microbiota, a key determinant of host physiology including nutrition and development. Larvae were reared on agri food by products (okara, brewer’s spent grain, potato peels, and potato sorting residues) alongside a standard diet. Growth performance was quantified, and gut microbial communities were profiled in terms of taxonomic composition and total bacterial load. Moreover, to enhance larval performance on potato wastes, four bacterial strains were administered via the substrate with two inoculations; heat inactivated counterparts were also provided to evaluate their contribution as a source of nutrients. Okara supported the highest bioconversion efficiency and larval performance, followed by brewer’s spent grain and potato sorting residues, whereas potato peels induced substantial mortality (57%). Substrate reduction ranged from 34 to 55%, with okara again yielding the highest rate. Different substrate dependent gut microbial community were detected, yet Enterococcaceae consistently dominated, albeit with reduced relative abundance in larvae fed with potato peels and potato sorting waste. These to latter substrates promoted also higher bacterial loads compared with the standard diet. Bacterial supplementation did not influence larval survival or final biomass yield, although significant differences in larval weight emerged after the second inoculation, particularly with active strains. Overall, the exploitation of agri-food by products represents a valuable strategy to enhance the sustainability of black soldier fly production. Further insights are required to elucidate how substrate influence the gut microbiota and, in turn, insect–microorganism interactions, ultimately supporting the development of probiotic assisted rearing strategies to optimise insect rearing and its health. Acknowledgement: Work funded by PRIN 2022 Project InProFarm (Insect Probiotic-assisted Farming: a promising tool to enhance edible insect health and performances), grant number 2022L4NJMK-Next Generation EU, Mission 4, Component 2.