Macronutrient balance differentially regulates growth, immune function, and oxidative stress responses in Plutella xylostella

Nutrition plays a fundamental role in insect physiology, yet how variation in dietary macronutrient balance shapes multiple physiological functions remains poorly understood. Using the Geometric Framework for Nutrition, we examined how dietary protein and carbohydrate composition influence growth, immunity and stress physiology in the specialist herbivore Plutella xylostella . Growth depended jointly on protein and carbohydrate availability, whereas immune and oxidative stress traits showed distinct response surfaces across the tested nutrient landscape. Total hemocyte counts and phenoloxidase activity were highest at intermediate protein levels, and survival following Bacillus thuringiensis challenge was likewise associated primarily with protein availability, pointing to a shared nutritional basis for immunity and pathogen resistance. Antioxidant and detoxification enzymes showed trait-specific nutrient responses, indicating that different physiological systems are favored by different macronutrient combinations. No single protein-carbohydrate ratio within the tested landscape simultaneously supported growth, immune performance, and oxidative homeostasis, highlighting trade-offs associated with nutrient allocation across physiological systems. These findings advance understanding of how nutrient-dependent mechanisms coordinate – and constrain – multiple physiological functions in a specialist herbivore.