Territory Preferences and Intensity of Competition in the Grey Hairstreak Strymon melinus (Lepidoptera, Lycaenidae) and the Tarantula Hawk Wasp Hemipepsis ustulata (Hymenoptera, Pompilidae)

Abstract

Males of the tarantula hawk wasp Hemipepsis ustulata and the grey hairstreak butterfly Strymon melinus have independently evolved highly similar reproductive tactics. Males of both species perch in and defend certain palo verde trees growing on ridgetops in central Arizona. The most popular trees, judging from frequency of occupation and frequency and duration of male-male aggressive encounters, are the same for both species. For both insects, most interactions between a territorial male and an intruder end quickly with the departure of the intruder. In the tarantula hawk, long aerial duels can be experimentally induced by capturing a resident male and then releasing him after a new male has taken control of his perch tree. Female tarantula hawks and female hairstreaks occasionally visit perch territories; although the data are scant, they suggest that females most often come to the territory location that males prefer. INTRODUCTION Many unrelated insect species use hilltops as mate-encounter sites (Shields, 1967). For example, over the course of a spring day in central Arizona, one can find males of several species of flies, wasps and butterflies perched on or patrolling near prominent shrubs and trees on hilltops that rise steeply from the surrounding desert plain. Among the hilltoppers are a large cuterebrid bot fly (Alcock and Schaefer, 1983), the California patch, a nymphalid butterfly (Alcock, 1985), two lycaenid butterflies, the great purple hairstreak and the grey hairstreak (Alcock, 1983a; Alcock and O'Neill, 1986), and a pompilid wasp (Alcock, 1979, 1981, 1983a). Males of all these insects are territorial. They defend perching sites on or near certain palo verde trees (Cerdidium microphyllum) that grow on the tops of ridges and hills. Although the palo verdes found on the highest points on ascending ridges seem to be the most consistently occupied by territorial hilltopping insects, other locations all along the backbone of a ridge are also claimed at times. Differences in the frequency with which individual trees are occupied provide a way to measure the preferences of hilltopping males for particular perches. Frequency of occupation data collected over several years for the tarantula hawk wasp Hemipepsis ustulata and the great purple hairstreak Atlides halesus have revealed that: (1) the same trees are claimed year after year by different generations of males of the two species; (2) the perch rankings remain much the same from year to year within a species, and (3) the wasp and hairstreak rank the trees in a similar manner, occupying the same few trees most consistently during their flight season (Alcock, 1983b). The question is, are the detailed behavioral similarities between Hemipepsis ustulata and Atlides halesus an exceptional case, or is convergence among territorial hilltopping insects widespread? We explored this question by comparing the tarantula hawk with another butterfly, Strymon melinus, a hairstreak that is not particularly closely related to A. halesus. Alcock and O'Neill (1986) established that males of both species perch in certain ridgetop palo verdes and that chases and ascending spiral flights between residents and conspecific intruders occur in both wasp and butterfly. This paper examines whether the wasp and butterfly also share similar perceptions of territory quality, which we measured by examining: (1) the frequency of occupation of palo verdes growing on 'Present address: Department of Entomology, Montana State University, Bozeman 59717.