Resistance to a multi-host parasite; it’s good to be rare

Closely related host species are known to show variation in the level of resistance towards the same or similar parasite species, but this phenomenon is largely unstudied. In this study, we examine the expression of resistance of two closely related species of damselflies (Nehalennia irene and N. gracilis) against Arrenurus sp. water mites, from two isolated sphagnum bogs. We show that both host species have statistically indistinguishable measures of parasitism (both prevalence and intensity) by larvae of a single mite species. Even though both species had the same levels of parasitism, the regionally rare host (N. gracilis) completely resisted the parasite in a novel way, whereas, the more regionally widespread species (N. irene) did not resist any of the parasites. These results suggest a strong historical selection from this mite on N. gracilis in a closed system.