Considerable attention has been paid to negative impacts of invasive species on disease, yet little research has investigated positive impacts invasive species can have on native host-parasite dynamics. When/where infectious stages are ubiquitous and native host-pathogen specificity is strong, invasive non-competent hosts can reduce native host-parasite encounter rate, effectively “diluting” the pool of infectious stages. Alternatively, invasive species could alter transmission via changes in native species abundance. Research suggests that environmental factors can strongly impact disease dynamics by altering host or parasite condition. Little is known about potential interactive effects of invasion and environmental factors on native species disease dynamics. We tested the hypotheses that 1) the invasive mosquito, Aedes japonicus, would dilute the prevalence of the parasite Ascogregarina barretti in the native mosquito Aedes triseriatus and 2) that the effect may be mediated by resource availability. We established larval habitat mesocosms in a 2x2 factorial field experiment with Ae. japonicus removal (yes or no) and resource level (high or low). We removed, identified, and returned the larval community to mesocosms weekly to determine species composition and densities. After nine weeks, we dissected all late instar Ae. triseriatus and a sample of Ae. japonicus to determine parasite prevalence and burden.
Removal of Ae. japonicus had no effect on Ae. triseriatus density. We detected a significant interaction between resource level and collection date on Ae. japonicus density with higher densities in the low resource habitat early in the experiment, and shifting to higher densities in the high resource habitat through time. Ae. triseriatus densities were higher in the low resource habitats throughout the experiment. We detected a significant interaction between Ae. japonicus removal and resource level on As. barretti prevalence in Ae. triseriatus. Parasite prevalence increased significantly when Ae. japonicus was removed and resources were high, despite lower Ae. triseriatus densities and expected encounter rates in the high resource treatment. We found no significant effects on parasite burden. We conclude that an invasive species can facilitate a native species by reducing parasite burden via a “dilution effect” and that these effects can be modified by environmental conditions.