Macronutrient balance generates distinct nutritional optima for growth, immune function and stress responses in the specialist herbivore Plutella xylostella

Nutrition plays a fundamental role in insect physiology, yet whether growth, immunity and stress responses share a common nutritional optimum remains poorly understood. Using the Geometric Framework for Nutrition, we investigated how dietary protein and carbohydrate balance regulate multiple physiological functions in the specialist herbivore Plutella xylostella. Growth was jointly determined by protein and carbohydrate intake, with maximal biomass accumulation constrained primarily by protein availability. Immune traits showed distinct nutritional optima: total hemocyte counts and phenoloxidase activity were maximised at intermediate protein-carbohydrate (P:C) ratios, whereas survival following Bacillus thuringiensis challenge depended predominantly on protein intake. Antioxidant and detoxification responses likewise varied non-linearly across the nutrient landscape, with individual enzyme systems each exhibiting distinct nutrient-dependent optima. Importantly, no single macronutrient balance simultaneously maximised growth, immune performance and oxidative homeostasis, demonstrating that these functions have divergent nutritional requirements. Together, these findings show that macronutrient balance generates competing physiological demands, offering an integrated view of nutrient-dependent regulation of growth, immunity and stress physiology in a specialist herbivore.