Parasitism risk and infection alter host dispersal.
Dispersal determines the spatial dynamics of host-parasite assemblages, particularly during invasions and disease epidemics. The risk of parasitism may create an incentive for dispersal, but infection is expected to reduce dispersal ability, which may alter the host's dispersal response to biotic stressors, including population density. We measured the dispersal of a semiaquatic insect (Notonecta undulata) in aquatic mesocosms in which we manipulated the presence of ectoparasitic Hydrachnidia mites and infected conspecifics. We found that parasitism risk increases host dispersal propensity. Using a flight assay, we determined that parasite infection reduces host dispersal ability. Finally, we used a mark-release-recapture study to investigate the joint effects of both parasitism risk and parasite infection on host dispersal in a natural, spatially structured population. We found that parasitism risk reduced dispersal probability, eliminated positive density-dependent dispersal, and increased dispersal distance. Infection had no effect on dispersal in the natural population. Our results show that parasites can both increase and decrease the movement rates of their hosts, depending on the ecological context, and can alter the host's dispersal response to other biotic stressors. Future studies should consider the consequences of this double-headed impact of parasites for landscape connectivity, population persistence, and host-parasite coevolution.