Ectoparasitism varies with latitude and likely to expand range under future climates

Climate change driven range expansion of parasites may increase parasite prevalence and intensity in host population and thereby reducing host fitness and contribute to the decline of host populations. Besides host and parasite genotype, local climate is an important contributor of the variation in parasitism. Here, we are interested in predicting parasitism in insect populations in future climates by mapping host-parasite interactions across a temperature and latitudinal gradient. We test the assumption that parasitism is more abundant in the tropics by analysing the spatial variation and effects of climatic factors on the prevalence and intensity of ectoparasitic water mites (Arrenurus sp.) and endoparasitic protozoans (Apicomplexa: gregarine) on Australian common bluetail damselflies (Ischnura heterosticta). We collected damselflies from 43 different populations across an extensive latitudinal gradient along the east coast of Australia covering ~18° of latitude and three distinct climatic zones: tropics, subtropics and temperate. In support of our predictions, we found that water mite prevalence and intensity were higher in the tropics than in subtropical and temperate biomes. This trend was only supported in males but not females for gregarine parasitism. We found that water mite prevalence and intensity but not gregarine correlated with latitude. Bioclimatic analysis suggests that annual temperature is the main driver for water mite prevalence, and precipitation is the main driver for water mite intensity whereas gregarine parasitism was unaffected by these climate variables. Based on these data, our future climate analysis suggests that ectoparasitism is likely to extend their range in this species under a warming climate. Considering the fitness cost imposed by parasites, the expansion of parasitism is likely to contribute to the decline of already vulnerable populations.