Diffuse coevolution: constraints on a generalist parasite favor use of a dead-end host

Many evolutionary models and empirical studies of parasite‐host interactions consider single species of parasites exploiting single host species. However, many parasites are generalists in that they parasitize more than one host species (often many more) and establish associations with other hosts that cannot be described as true parasitism. We identify such an association, explain how constraints may maintain it, and indicate why such diffuse interactions are deserving of attention. We describe the use of two closely related Sympetrum dragonfly species by larvae of the water mite Arrenurus planusMarshall. Adults of one dragonfly species are resistant whereas adults of the other species are almost wholly susceptible to A. planus. However, A. planus attaches as often to the resistant host as it does to the susceptible host species when relative abundance and seasonal timing of adult emergence of both species is considered. We present evidence that mites track the susceptible host and are most active early in the season, when early‐emerging unsuitable hosts are also present. Thus, use of resistant hosts appears an unavoidable outcome of constraints promoting discovery and use of susceptible hosts. Such findings have implications for studies of local adaptation and host switching.