The host-parasite relationship is a primary driver of coevolution, influenced by multiple environmental factors. The abundance of parasites in the habitat can impact the frequency with which hosts encounter parasites, affecting both infection prevalence and parasite burden within the host. Potential hosts with low potential for infection could alter this relationship by reducing the abundance of available parasites in the habitat via dilution. The mosquito, Aedes triseriatus, is native host to protozoan parasite Ascogregarina barretti. Aedes albopictus, an invasive competitor of A.triseriatus, has the potential to be a diluting host for a species-specific parasite like A. barretti. We hypothesized that 1) A. barretti infection prevalence in A. triseriatus would be positively correlated with parasite abundance and 2) A. triseriatus survivorship would be negatively correlated. Additionally, 3) A. albopictus will dilute infection prevalence in A. triseriatus. Aedes triseriatus larvae were exposed to 4 dosages of A. barretti. To investigate potential for dilution, A. albopictus was given standardized dosages of A. barretti, then A. triseriatus were exposed to water wherein A. albopictus had: grazed on A. barretti; developed after grazing; or died as adults after grazing. Randomly selected A. triseriatus larvae and adults were dissected to determine infection prevalence and burden.
We detected a significant effect of dosage on parasite burden, which was positively correlated with parasite abundance. We found a significant effect of dosage on infection prevalence, which increased significantly at higher dosages. We detected a significant effect of dosage on survivorship to adulthood, with increased survivorship in cohorts from the highest dosage. Multivariate analysis indicated that infection prevalence contributes most to differences among dosage groups, and is positively correlated with survivorship and parasite burden. We detected no significant effect of dosage on gametocyst abundance, with few individuals containing gametocysts. All cohorts in dilution treatments with A. albopictus experienced significantly reduced larval parasite burden and infection prevalence compared to non-dilution treatment. This indicates that A. albopictus is acting as a dilution host for A. barretti probably by removing viable oocysts from the parasite population. We conclude that an invasive host may reduce parasite abundance in the habitat, leading to a decrease in native host infection prevalence and burden, a dilution effect. These preliminary data indicate that lower parasite abundance in the habitat, reduced infection intensity, and reduced parasite burden may not be associated with increased native host survivorship.