Bioconversion of agri-food by-products through Hermetia illucens larvae and evaluation of probiotic administration

The larvae of Hermetia illucens L. (Diptera: Stratiomyidae), also known as the black soldier fly, are capable of converting low-value by-products into high-quality biomass, rich in proteins and lipids, thus contributing to their valorization and reuse within the circular economy. Larval biomass can be used for different aims, but its use as feed for livestock (fish, poultry, and pigs) is of considerable interest today, given the growing demand for protein sources (particularly fish meal and soy) and their rising prices. However, to ensure sustainable and low-cost mass production of the insect, it is essential to identify rearing substrates that are widely available and low-cost. In this study, different by-products from the agri-food industry were considered for rearing black soldier fly larvae, and their growth and substrate bioconversion efficiency were evaluated. Additionally, to ensure better larval performance, the influence of administering probiotic bacteria previously isolated from the gut of H. illucens and other dipterans was explored. The by-products considered were: brewers' spent grain, okara (by-product from soy milk production), potato selection waste, and potato peels. The best results, both in terms of growth (number of developmental days and larval weight) and survival, were obtained on okara, followed by brewers' spent grain and potato selection waste, while potato peels led to a high mortality rate (57%). Substrate reduction ranged from 34% to 55%, with the best performance observed on okara. Subsequently, to optimize larval growth on potato selection waste, four different probiotic strains were provided to the larvae through the rearing substrate in two consecutive administrations one week apart. Heat-inactivated strains were also administered to evaluate their possible effect on the host performance as a source of nutrients. Supplementation with bacteria did not affect larval survival or the final biomass produced, although statistical differences were observed in the weights reached by the larvae after the second administration, particularly with the active forms of some strains. The bioconversion capacity was not affected by the administered probiotics. Further investigations are needed to evaluate the best way and dose of administration, including other bacterial strains, and to consider their contribution in preventing potential infections in black soldier fly mass rearing.