Host-parasite interactions involve competition for nutritional resources between hosts and the parasites growing within them. Consuming part of a host's resources is one cause of a parasite's virulence, i.e. part of the fitness cost imposed on the host by the parasite. The influence of a host's nutritional conditions on the virulence of a parasite was experimentally tested using the mosquito Aedes aegypti and the microsporidian parasite Vavraia culicis. A condition-dependent expression of virulence was found and a positive relation between virulence and transmissibility was established. Spore production was positively influenced by host food availability, indicating that the parasite's within-host growth is limited by host condition. We also investigated how the fitness of each partner varied across the nutritional gradient and demonstrated that the sign of the correlation between host fitness and parasite fitness depended on the amount of nutritional resources available to the host.

Abstract. 1. Due to its effects on the phenotypic and genotypic expression of lifehistory traits, density-dependent competition is an important factor regulating the growth of populations. Specifically for insects, density-dependent competition among juveniles is often associated with increased juvenile mortality, delayed maturity, and reduced adult size.2. The aim of the work reported here was to test whether the established phenotypic effects of density-dependent competition on life-history traits could be reproduced in an experimental design requiring a minimal number of individuals. Larvae of the mosquito Aedes aegypti were reared at densities of one, two, or three individuals per standard Drosophila vial and in six different conditions of larval food availability. This design required relatively few individuals per independent replicate and included a control treatment where individuals reared at a density of one larva per vial experienced no density-dependent interactions with other larvae.3. Increased larval densities or reduced food availability led to increased larval mortality, delayed pupation, and the emergence of smaller adults that starved to death in a shorter time (indicating emergence with fewer nutritional reserves).4. Female mosquitoes were relatively larger than males (as measured by wing length) but males tended to survive for longer. These differences increased as larval food availability increased, indicating the relative importance of these two traits for the fitness of each sex. The role of nutritional reserves for the reproductive success of males was highlighted in particular.5. This minimalist approach may provide a useful model for investigating the effects of density-dependent competition on insect life-history traits.