Larval nutrition reshapes growth-immunity trade-offs and produces region-specific changes in adult morphology.

Resource availability during development is a key driver of physiological trade-offs, yet how early nutritional environments shape allocation strategies across life stages remains unclear. We examined how larval diet quality influences trade-offs among immune investment, growth and adult morphology in the hawkmoth Hyles lineata (Sphingidae). Larvae reared on higher-quality diets exhibited enhanced immune responses, along with faster growth and greater body mass. However, immune investment varied by component: total haemocyte count (THC) increased under both medium- and high-quality diets, while phenoloxidase (PO) activity was elevated only under high-quality diets. Body mass did not predict THC, and a negative relationship emerged between body mass and PO activity under high-quality diets, suggesting context-dependent allocation trade-offs. Larval diet had lasting effects on adult morphology: higher-quality diets led to increased thorax and abdomen sizes, while head size remained unchanged. Interestingly, a negative correlation emerged between larval immune investment and adult body structure, where elevated larval PO activity was associated with reduced adult thorax and abdomen sizes, revealing a cross-stage trade-off on high-quality diets. Mild larval stress further altered these patterns in a diet-dependent manner. On the low-quality diet, stressed larvae developed larger thorax and abdomen than controls, whereas on the high-quality diet, stress activation reduced or erased size gains.