Mounting effective resistance against pathogens is costly in terms of energy and nutrients. However, it remains unexplored whether hosts can offset such costs by adjusting their dietary intake so as to recoup the specific resources involved. We test this possibility by experimentally challenging caterpillars (Spodoptera littoralis) with a highly virulent entomopathogen (nucleopolyhedrovirus), under dietary regimes varying in the content of protein and digestible carbohydrate. We found that dietary protein influenced both resistance to pathogen attack and constitutive immune function to a greater extent than did dietary carbohydrate, indicating higher protein costs of resistance than energy costs. Moreover, when allowed to self-compose their diet, insects surviving viral challenge increased their relative intake of protein compared with controls and those larvae dying of infection, thus demonstrating compensation for protein costs associated with resistance. These results suggest that the change in the host's nutritional demands to fight infection induces a compensatory shift in feeding behaviour.

Summary 1.In insects, cuticular melanization and immune function are strongly dependent on the quantity of dietary protein ingested. However, relatively little is known about the role played by the quality of nitrogenous resources in determining phenotypic variation in the degree of melanization and correlated immunological functions. We explored this issue in a generalist-feeding caterpillar, Spodoptera littoralis , by providing larvae with one of two semi-artificial diets differing in their quality of protein supplement (high-quality casein vs. low-quality zein). 2. Larvae given a high-quality protein diet had higher survival and faster growth rates than larvae on the low-quality protein diet; they also had more heavily melanized cuticles. Two components of constitutive immunity were assayed: lysozyme-like antibacterial activity and phenoloxidase (PO) activity. PO activity was not affected by diet quality, but antibacterial activity was higher for insects on the high-quality diet, providing a potential physiological mechanism for observed survival differences between the two dietary treatments. 3. Analysis of nitrogen conversion efficiency using chemically defined diets indicated that proteinquality had little effect on ingestion rates (i.e. nitrogen acquisition), but that post-ingestive utilization of nitrogen was reduced for larvae on the low-quality protein diet. This result implies that proteinquality had a significant influence on the nitrogen pool potentially available for investment in melanin production and immune function. 4. A split-plot, full-sib family breeding experiment was used to dissect the genetic control of cuticular melanization from the effects induced by dietary treatment. Estimates of broad-sense heritability indicated that the expression of melanization had a significant genetic basis, but there was little evidence for a genotype × environment interaction. 5. These results suggest that nutrition is a key factor that influences insect melanization and mediates its coupling with important physiological functions linked to survival.