Benefits of Dispersal in Patchy Environments: Mate Location by Males of a Wing-Dimorphic Insect

Dispersal dimorphisms, in which both flight-capable and flightless adults occur in the same species, are commonplace in insects. Such dimorphisms are seen as reflecting a balance between the benefits and costs of flight and wing reduction or loss. In heterogeneous habitats, theory predicts that fitness trade-offs can favor the evolution of a dispersal dimorphism in which both flight-capable and flightless morphs are retained in the same population. Despite the wealth of theory, however, there has never been an explicit field assessment of how habitat heterogeneity directly influences the reproductive success of the flight-capable and flightless wing forms of an insect species. The objective of this research was to investigate how variation in habitat heterogeneity (vegetation structure) and female density influence mate location and thus the potential for reproductive success by the male wing forms of a salt marsh inhabiting insect, the planthopper Prokelisia dolus. By placing unmated females in the field at different densities and in sparse and contiguous vegetation, we were able to compare the ability of male wing forms to locate stationary mates. Our data show that both vegetation structure and female density differentially influenced the ability of the male wing forms to locate mates. Flight-capable males located females and acquired matings far more frequently in sparse vegetation, whereas flightless males discovered females more often in contiguous vegetation. Flight-capable males located females more effectively at low female densities, whereas flightless males discovered females more efficiently at higher female densities. Thus, natural variation in vegetation structure and female density are two important factors which combine to influence the mating success of each male wing form. We conclude that habitat heterogeneity in concert with the known reproductive penalties imposed by flight capability (reduced siring ability) interact to favor the persistence of the dispersal polymorphism in males of this planthopper species.