Cannibalism facilitated by parasite infection induces dispersal in a semi-aquatic insect.

Parasites are known to have direct effects on host dispersal ability and motivation. Yet, parasites have a variety of impacts on host populations, including shaping predation and cannibalism rates, and therefore may also have indirect effects on host dispersal; these indirect pathways have not been studied. We tested the hypothesis that parasites influence host dispersal through effects on cannibalism using backswimmers ( Notonecta undulata ) and Hydrachnidia freshwater mites. Mite parasitism impedes swimming in backswimmers, which we found increased their vulnerability to cannibalism. We imposed a manipulation that varied cannibalism rates across experimental populations consisting of a mix of backswimmers with and without simulated parasites. Using simulated parasites allowed us to examine the effects of cannibalism without introducing infection risk. We found that the odds of dispersal for infected backswimmers increased by 2.25× with every 10% increase in the risk of being cannibalized, and the odds of dispersal for healthy backswimmers increased by 2.34× for every additional infected backswimmer they consumed. Our results suggest that cannibalism was used as an energy source for dispersal for healthy individuals, while the risk of being eaten motivated dispersal in infected individuals. These results elucidate the complex ways that parasites impact host populations and strengthen our understanding of host-parasite interactions, including host and parasite population stability and spread. This article is part of the theme issue 'Diversity-dependence of dispersal: interspecific interactions determine spatial dynamics'.