Life History and Social Evolution of the Primitively Eusocial Bee Augochlorella striata (Hymenoptera: Halictidae)

Life history parameters of the primitively eusocial bee Augochlorella striata (Hymenoptera: Halictidae) are summarized for a population in central New York, and analyzed with respect to the evolutionary maintenance of eusociality in this species. High mortality (39%) of solitary foundresses and low mortality (12%) of social colonies indicate severe ecological constraints limiting independent (solitary) reproduction. Extreme relat edness asymmetries (3:1, due to singly-mated queens) and female-biased reproductive broods create genetic incentives favoring worker behavior (helping). In addition, workers appear to capitalize on relatedness asymmetries by biasing the sex ratio towards their more closely related sisters and away from their more distantly related brothers, because eusocial colonies (headed by a mother queen; relatedness asymmetry present) consistently produce more female-biased sex ratios than parasocial colonies (headed by a sister queen after queen supersedure; relatedness asymmetry absent). Both intrinsic factors inherent in the haplo diploid system of sex determination (relatedness asymmetries in conjunction with female biased sex ratios) and extrinsic factors (ecological constraints) therefore appear to modulate eusocial evolution in A. striata. Augochlorella striata (Provancher) is an ubiquitous bee in eastern North Amer ica, ranging from Texas to Nova Scotia and from the central plains to the east coast (Ordway, 1966a, b; Packer Jessome et al., 1989). It is the northernmost representative of the Augochlorini, a strictly New World tribe of mostly brilliantly metallic green-blue bees with the center of distribution in the Neotropics (Eick wort, 1969). Most augochlorine bees are solitary or semisocial, but bees in one clade that includes the genus Augochlorella are eusocial (Danforth and Eickwort, in press). The annual life cycle of A. striata follows the typical pattern of primitively eusocial, temperate halictine bees (Ordway, 1966a, b; Michener, 1990; Packer, 1993). During the foundress phase in spring, foundresses construct a nest in the soil and provision a first brood of both sexes. The emergence of the first brood in early summer marks the beginning of the worker phase, as first-brood females remain with their mother as workers. Foundresses now cease all foraging, become queens, and, with the help of their workers, produce a second brood of male and female reproductives. The emergence of the second (reproductive) brood in late summer marks the beginning of the reproductive phase. Reproductives mate in late summer and early fall, but only inseminated females overwinter to complete the colony cycle. In some colonies, the foundress dies during the worker phase, and one of her daughters assumes the position of a replacement queen. Colonies undergoing queen supersedure are called "parasocial" (queen and workers are from the same generation), to distinguish them from "eusocial" colonies (queen and workers are from different generations) that are headed by the foundress queen throughout the season. 1 Current address: Department of Zoology, University of Maryland, College Park, MD 20742 USA. VOLUME 69, NUMBER 4 117 Because of its eusocial life history and abundance, A. striata had been selected for an experimental study of queen-worker conflict over the colony sex ratio (Mueller, 1991; Mueller et al., 1994). This paper summarizes the life history of A. striata in central New York, compiled over the four seasons (1989 to 1992) when the experiments on sex-ratio conflict were conducted. Previous reports on this bee include accounts of nesting and nest architecture (Knerer and Atwood, 1962; Ordway, 1961; Ordway, 1966a; Packer, Sampson et al., 1989), systematics (Ordway, 1966b; Eickwort, 1969), natural enemies (Ordway, 1964), caste differ entiation (Ordway, 1965; Packer, Jessome et al., 1989), allozyme variation (Packer and Owen 1990), and intrapopulation variation in social organization (Packer