Interaction effects of substrate fermentation and larval density on black soldier fly life-history traits

Agricultural by-products can serve as an excellent food source for edible insects, but their high-fibre properties can present challenges. One solution to this is fermentation, which can enhance their nutritional value by breaking down the fibre. However, little research has been conducted on how this method interacts with other environmental factors in insect rearing. To address this gap, our study aimed to investigate the impact of substrate fermentation and larval density on black soldier fly (BSF) larvae. We compared fermented substrates (fermented spent grain and additionally fermented ensiled grass) with standard fibrous substrates (spent grain and ensiled grass) and applied two larval density treatments (high and low). Our findings revealed that prepupal mass was significantly greater in fermented substrates than in standard fibrous substrates, with variations dependent on the substrate and larval density treatments. Larval density significantly influenced prepupal mass only in the fermented spent grain treatment. Substrate type influenced development time, with fermented spent grain resulting in a shorter development time than ensiled grass. However, substrate fermentation and larval density did not affect development time. Substrate fermentation only increased larval survival when individuals were reared on spent grain at high larval density. There were no significant differences in survival between fermented and standard substrates in other substrate and larval density combinations. Our study demonstrates that fermentation could serve as a way to amend fibrous substrates, making them suitable for rearing BSF larvae; however, its effects depend on environmental factors such as larval density.