Flexible diet choice offsets protein costs of pathogen resistance in a caterpillar

Abstract

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.

Figure 1

Effects of NPV challenge in relation to dietary protein-to-carbohydrate ratio (P : C) on (a) survival, (b) development rate (for the reciprocal of stadium duration), and (c) percent nitrogen content of lipid-free dry pupae of caterpillars in the no-choice experiment. In (b) and (c), the bar indicates the mean value (±1 s.e.m.).

Figure 2

Composite performance estimate of NPV challenged and control caterpillars in the no-choice diet experiment. These values (means±1 s.e.m.) are calculated by multiplying survival by average biomass gain per day (mg day⁻¹). Each descending arrow indicates pathogen-induced performance loss of insects feeding on each P : C diet.

Figure 3

Dietary influence on constitutive immune function variables (means±1 s.e.m.): (a) lysozyme-like antimicrobial activity, (b) encapsulation response, (c) phenoloxidase (PO) activity, and (d) total haemocyte count (THC). Data from the two highest (35 : 7 and 28 : 14) and two lowest (14 : 28 and 7 : 35) P : C ratios were pooled for illustrative clarity.

Figure 4

Proportions of protein and carbohydrate (P : C ratio) selected by caterpillars over the days post-infection in the choice diet experiment (means±1 s.e.m.). The daily selection ratios were compared between (a) NPV challenged and control insects, and between (b) surviving and dying insects challenged with NPV (^*0.01<p<0.05, ^**0.001<p<0.01).The dotted lines indicate the selected P : C ratio of 2 : 1 if feeding had occurred randomly between the two choice diets (35 : 7 and 21 : 21).