An experimental study of the attractiveness of artificial perch territories to male tarantula-hawk wasps, Hemipepsis ustulata (Hymenoptera: Pompilidae)

Males of the tarantula-hawk wasp Hemipepsis ustulata (Dahlborn) defend perch sites on the highest points of prominent hills and ridges in central Arizona (Alcock 1979, 1981, 1983). A territorial male permits no other male to alight in his plant, be it a palo verde (Cercidium microphyllum), jojoba bush (Simmondsia chinensis), creosote bush (Larrea tridentata), saguaro cactus (Carnegia gigantea), or cholla cactus (Opuntia spp.). Receptive females sometimes visit a territory site; mating occurs when the resident male captures the female in flight and the pair descends to the ground for a brief copulation. Because defended plants contain no food or oviposition resource for females nor any nests from which virgin females may emerge, males of H. ustulata appear to practice lek territoriality (Alcock 198 l); they defend areas whose only value to females is the presence of a mating partner. A striking feature of male behavior is the consistency with which individuals of different generations select certain perch trees from year to year. In one study site many dozens of palo verdes grow on the ridge and yet, every year over five flight seasons the same two trees have been most frequently occupied by territorial wasps (Alcock 1984). A few other trees are almost as popular while some are usually claimed for only a few days of the 2-3 month flight season. The wasps completely ignore the many other trees on the ridge. We have analyzed the properties of natural territories in order to determine why some trees are so much more likely to be defended than others (Alcock 1984). Males only defend palo verdes growing