Consequences of dietary amino acid imbalance on growth and gas exchange in black soldier fly (Hermetia illucens) larvae

Black soldier fly larvae (Hermetia illucens (L.); BSFL) production relies on efficient nutrient conversion into protein-rich biomass to ensure sustainability and economic viability. While the effects of dietary protein content and macronutrient ratios are well described, the metabolic costs of providing an imbalanced essential amino acid (EAA) supply in dietary proteins remain poorly understood. We examined how dietary EAA imbalance affects growth, metabolism, and energy allocation in BSFL. Larvae were reared on a reference diet (chicken feed) and three isonitrogenous, semi-synthetic diets: one balanced diet matching the EAA profile of the reference, and two unbalanced diets selectively deficient in either leucine, phenylalanine, and threonine or isoleucine, lysine, and tryptophan. Using open flow respirometry, daily growth monitoring, and biochemical analysis, we quantified gas exchange, body composition, and the energetic costs of biomass and protein deposition. EAA imbalance delayed development, reduced growth and protein content, and promoted lipid accumulation, while O2 consumption and CO2 production were reduced and respiratory exchange ratios exceeded 1.0, indicating elevated lipogenesis. Metabolic costs per unit protein deposited were higher on EAA-deficient diets (up to 3.63 kJ per kJ protein), reflecting inefficient amino acid utilisation. In contrast, the balanced diet supported more efficient protein deposition at a lower energetic cost (2.73 kJ per kJ protein). Our findings demonstrate that suboptimal dietary EAA profiles reduce metabolic efficiency in BSFL. Quantifying the physiological costs of amino acid imbalance provides benchmarks to guide feed formulations in insect production systems to minimise input costs and maximise the nutritional quality and quantity of larval output.