Effects of population density on immunity and reproduction in bean beetles Callosobruchus maculatus

In bean beetles (Callosobruchus maculatus), population density affects larval development. Under high density, individuals develop into a dispersal morph, with larger wings and smaller gonads, while under low density we observe the flightless morph with smaller wings and larger gonads. Past research has shown that the effect of density on wing size is stronger in males than in females, and we hypothesized that this difference is caused by a sex-specific trade-off between immunocompetence and reproduction. We predicted that females reared under high larval density would have an overall stronger immune response compared to those reared in low densities, and to males in general. We tested this hypothesis by rearing males and females under different density treatments and measuring wing size, gonad size, and immune response through phenoloxidase and lytic activity. Females reared under the high-density treatment displayed stronger immune responses, and a trade-off between immunity and gonad size. This provides evidence for a sex-specific, density-specific trade-off between immunocompetence and reproduction.