Growth and metabolic performance of house fly and black soldier fly larvae differ across densities and waste-based growth substrates

Large scale production of insect larvae is considered a sustainable way to upcycle various organic waste- and by-products into more valuable food and feed products. The sustainability of insect larvae production depends on the substrates and species being used, but comparative studies that include both growth and efficiency are lacking. Here we compare larval fitness, including survival, development time, weight, substrate conversion efficiency, substrate reduction, and metabolic parameters across different combinations of densities and waste- and by-product-based substrates on the two fly species, the house fly (Musca domestica) and the black soldier fly (Hermetia illucens). The waste- and by-product-based substrates were a brewer's spent grain-based substrate, a digested sludge-based substrate, and a wheat bran/deproteinized grass-based substrate all highly abundant and of low value. Substrate and density significantly impacted on most larval growth and metabolic performance traits, but dependent on species. The brewer's spent grain-based substrate generally gave the highest performance in terms of larval weight, larval yield, and substrate conversion efficiency for both species, while a high density gave a higher larval yield and substrate conversion efficiency, but lower larval weight. Generally, black soldier fly larvae showed lower metabolic costs and higher net growth efficiency than house fly larvae. Altogether, our results demonstrate that both larval species, substrate, and larval densities affect larval growth and metabolic performance, and subsequently the scope for valorizing waste- or by-products to achieve a sustainable production of food and feed.