The dilution effect in a field survey of eastern mosquitoes

The dilution effect is the hypothesis that as diversity of species in a community increases, the prevalence of a disease decreases.  This effect has been observed in the field, though such research has focused primarily on the Lyme disease-deer tick system, and it is unknown how broadly applicable the dilution effect is.  Here we describe the results of the first year of an extensive field survey of container-dwelling mosquitoes and their Ascogregarina parasites.  Ascogregarina are protest gut parasites with potential importance as biological control agents of mosquitoes, while their hosts, container-dwelling mosquitoes, are important vectors of disease. We identified eight different species of mosquitoes from twelve field sites in North America, and at least two species of Ascogregarina infecting those mosquitoes.  Each of our twelve sites had between one and five samples from different container habitats.  We used three measures of richness in order to examine the dilution effect at different scales.  Using a bootstrapped regression between richness measurement and disease prevalence, we found support for the dilution effect at the site-based level of scale.  This suggests that the dilution effect may be playing a role in natural systems such as this mosquito-gregarine system, but that scale is an important consideration in how broadly applicable the dilution effect may be.