Relationship between substrate composition and larval weight: a simple growth model for black soldier fly larvae

Black soldier fly larvae (BSFL) could play an important role as a high-quality source of protein, lipid, and other nutrients not only for animal feed but also for human consumption. However, feeding material costs for the larvae strongly influence the economic feasibility of the feed source. Therefore, the performance of several side streams originating from food production, which are legally permitted in European Union as feed grade substrates, were tested as feed in rearing BSFL. The substrates differed in their composition with respect to macronutrients, like proteins and lipids, and minor nutrients, like minerals. A commercial chicken feed substrate was used as a control, and it was replaced stepwise by the investigated substrate. The effect of the replacement on gained larval weight, survival rate, and feed conversion ratio was determined. Data were used to develop a model to predict larval weight based on the substrate contents of crude lipid and crude protein. The lipid content distinctly influenced the larval growth enabling a maximum weight gain at moderate lipid contents, whereas a very sharp decrease in larval weight gain was found for lower lipid contents in the substrate. On the other hand, influence of protein content on weight gain was linear with higher weights at higher protein contents. For the substrates included, a direct influence of the content of digestible carbohydrates on weight was not found demonstrating that this component was not the limiting growth factor in our study.