Reduced flight capability in British Virgin Island populations of a wing-dimorphic insect: the role of habitat isolation, persistence, and structure

1. The effects of habitat isolation, persistence, and host-plant structure on the incidence of dispersal capability (per cent macroptery) in populatio ns of the delphacid planthopper Toya venilia were examined throughout the B ritish Virgin Islands. The host plant of this delphacid is salt grass Sporo bolus virginicus. which grows either in undisturbed habitats (large expanse s on intertidal salt flats and around the margins of salt ponds, or small p atches of sparse vegetation on sand dunes along the shore), or in less pers istent, disturbed habitats (managed lawns). 2. Both sexes of T. venilia were significantly more macropterous in disturb ed habitats (77.1% in males, 12.5% in females) than in more persistent, und isturbed habitats (19.2% in males, <18 in females). 3. Males exhibited significantly higher levels of macroptery (26.9 +/- 7.6% ) than did females (2.0 +/- 1.7%), and per cent macroptery was positively d ensity dependent for both sexes in field populations. 4. There was no evidence that the low incidence of female macroptery in a s ubset of island populations inhabiting natural habitats (1.7 +/- 1.2%) was attributable to the effects of isolation on oceanic islands. The incidence of macroptery in British Virgin Island populations of T. venilia was not di fferent from that observed in mainland delphacid species existing in habita ts of similar duration. 5. Rather, the persistence of most salt grass habitats throughout the Briti sh Virgin Islands best explains the evolution of flight reduction in female s of this island-inhabiting delphacid. 6. Males were significantly more macropterous in populations occupying dune vegetation (37.6 +/- 9.8%) than in populations occupying salt flat-pond ma rgin habitats (7.6 +/- 5.6%). By contrast, females exhibited low levels of macroptery in both dune (0%) and salt flat-pond margin (< 1%) habitats. Var iation in salt-grass structure probably underlies this habitat-related diff erence in macroptery because flight-capable males of planthoppers are bette r able to locate females in the sparse structured grass growing on dunes. T his habitat-related difference in male macroptery accounted for the general ly higher level of macroptery observed in males than in females throughout the islands. 7. The importance of habitat persistence and structure in explaining the in cidence of dispersal capability in T. venilia is probably indicative of the key role these two factors play in shaping the dispersal strategies of man y insects.