Halictus rubicundus Christ is a facultatively social bee demonstrating solitary behavior in regions of cool climate, social behavior in areas of warmer climate, and a mixture of social and solitary behavior in marginal environments. In this article I compare original data to those from previously published studies of H. rubicundus in a variety of environments. I describe nest structure, nest density, bee size, reproductive output, sex ratio, and incidence of predators and parasites. From the patterns that emerge, I conclude that nest-site philopatry is important in structuring populations so that social behavior is favored over bivoltinism in warm climates. The sex ratio of populations is correlated with photoperiod but the relationship is not absolute; therefore, I propose that some other causal factor or a combination of factors, such as temperature or resource availability, is involved. The same mechanism may be responsible for changes in female size during the season. Differences in social behavior do not appear to provide a barrier to reproduction, and the presence of different forms does not imply incipient speciation.
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