Larval diet quality affects allocation tradeoffs in both larvae and adult stages of a moth

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.