Density-Related Migration in Planthoppers (Homoptera: Delphacidae): The Role of Habitat Persistence

We investigated the selective pressures associated with the possession of wings and migration in wing-dimorphic planthoppers (Homoptera: Delphacidae). Both migratory (macropterous) and flightless (brachypterous) forms occur. We found that levels of migration (% macroptery) in field populations decreased significantly as the persistence of their habitats increased. By using phylogenetically independent contrasts between congeners, we obtained the same result, which suggests that habitat persistence has influenced levels of migration independent of common ancestry. Also, there was a clear difference in the density-wing form response between species in temporary and persistent habitats. Macropterous forms were triggered at lower densities for species inhabiting temporary rather than persistent habitats. However, males of species in temporary habitats were macropterous at both low and high rearing densities; macroptery was positively density-dependent in the females of these species and in both sexes of species in persistent habitats. Our results suggest that habitat persistence influences the migratory capability of planthoppers in two ways: (1) by selecting for habitat escape, and (2) by dictating the availability of mates. In persistent habitats wings are less necessary for habitat escape and they are rarely required for mate location. As a consequence and because wings impose a reproductive penalty, flightlessness prevails. For species in temporary habitats, wings are favored in males to locate females at low colonizing densities and are favored in both sexes at high densities for reasons of habitat escape. We conclude that habitat persistence has played a major role in shaping patterns of migration in this group of sap-feeding herbivores.