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. During development, insect larvae must balance investment between growth and immune defense, but the downstream consequences of these allocation decisions for adult traits are not well understood. We examined how larval diet quality influences tradeoffs 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 hemocyte count (THC) increased under both medium- and high-quality diets, while phenoloxidase (PO) activity was elevated only under high-quality diet conditions, indicating a higher nutritional threshold. Interestingly, body mass did not predict hemocyte count, 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. More interestingly, a mild larval stress 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. Collectively, our results demonstrate that the larval nutritional environment shapes allocation strategies across ontogeny, with hidden tradeoffs between growth and immunity leaving enduring imprints on adult phenotype.