Visualising the nutritional performance landscapes for the black soldier fly, Hermetia illucens (Diptera: Stratiomyidae)

The black soldier fly (BSF), Hermetia illucens (Diptera: Stratiomyidae), has become one of the most economically important insects that can be a sustainable replacement for traditional feed for swine, poultry, and aquaculture. Despite its emerging importance, studies examining the nutritional requirement of BSF have been scarce. Here we used nutritional geometry approach to determine the separate and combined effects of dietary protein and carbohydrate content on multiple life-history traits and body composition linked to fitness in BSF. BSF larvae were reared on one of 32 chemically defined diets that varied in protein-to-carbohydrate ratio (P:C=1:16, 1:8, 1:4, 1:2, 1:1, 2:1, 4:1, or 8:1) and in protein plus carbohydrate concentration (P+C=60, 120, 180, or 240 g/l). Females developed more slowly into larger adults than males, but two sexes responded to dietary variation in protein and carbohydrate in a similar manner. The topography of the nutritional performance landscapes mapped for all measured traits differed significantly from one another, with the optimal P:C ratio being 1:1.71 for preadult survivorship, 1:1.56 1:1.11 for development time, 1:3.36 - 1:3.16 for body weight at adult emergence, and 1:1.83 - 1:1.40 for growth rate. Significant divergence in nutritional optima among these components of fitness indicates that the occurrence of fundamental life-history trade-off can be mediated by macronutrient intake. The index representing the preadult performance of BSF had a nonlinear relationship with dietary protein, with the peak centred at the intermediate P:C ratios of 1:1.43 - 1:1.37. The optimal P:C ratio was 1:2.12 - 1:1.70 for lean body weight and 1:14.14 - 1:10.82 for lipid content. Our results highlight the importance of the balanced composition of dietary protein and carbohydrate for optimising BSF performance and have implications for enhancing the mass production of this beneficial species.