DEMOGRAPHY AND RELATEDNESS IN MULTIPLE-FOUNDRESS NESTS OF THE SOCIAL SWEAT BEE, HALICTUS-LIGATUS

Female sweat bees in the species Halictus ligatus exhibit a wide range of reproductive roles, ranging from typically foundress or queen-like to typically worker-like. Nests are founded in spring and most are ha plometrotic, that is, founded by a single foundress. A few (up to 12%) are pleometrotic, founded by 2-6 foundresses. Variation in the propor tion of multifoundress nests from year to year and from place to place suggests an adaptive basis for pleometrosis. We studied the demograph ic and social characteristics of 23 pleometrotic nests in an aggregati on of 250-300 nests near Victoria, Ontario, in 1984, 1990, and 1991. I n pleometrotic associations, dominant foundresses behaved in a manner typical of mid-summer, haplometrotic queens, while subordinates behave d like mid-summer workers. Dominant foundresses tended to be larger th an subordinates. Pleometrotic nests were significantly more likely tha n haplometrotic nests to produce brood, and they also produced more wo rkers. However, this early advantage did not result in the production of more reproductive brood per nest, nor did pleometrotic foundresses experience higher productivity per foundress than did haplometrotic fo undresses. Relatively low relatedness among various categories of broo d implied that subordinate foundresses were not closely related to dom inants. We suggest that pleometrosis most likely results from accident al encounters between spring foundresses as they leave their hibernacu la. Once formed, such associations confer a survival advantage on the nest as a whole, but do not result in greater reproductive brood produ ctivity.