High larval density does not induce a prophylactic immune response in a butterfly

Immune function is greatly affected by biotic and abiotic factors, arguably owing to resource allocation trade-offs. While starvation typically exerts negative effects on immune function, there are two competing hypotheses regarding the impact of population density: a high population density may decrease or increase immune competence, either as a result of reduced energy stores or of augmented resource allocation to minimise infection risk (density-dependent prophylaxis). Against this background, the effects of population (= larval rearing) density and transient food stress on adult immune function and life-history traits in the temperate-zone butterfly Pieris napi (Linnaeus) have been examined. Both a period of larval starvation and high larval rearing density prolonged the development time and reduced the body mass. All immune parameters measured (pro-phenoloxidase and phenoloxidase activity, haemocyte number, encapsulation rate) were also negatively affected by high density, whereas starvation had no effect. We thus found no evidence for density-dependent prophylaxis, but a diminished performance at a high density probably caused by reduced energy stores. The present study reinforces that the maintenance and deployment of an efficient immune system is costly, which may prohibit prophylactic responses to high population densities.