Social polymorphism and social behaviour in sweat bees (Hymenoptera: Halictidae)

I use field observations, experiments and molecular techniques to describe the social biology of the sweat bee Lasioglossum calceatum, and to investigate the mechanisms underlying social polymorphism and body size in this species. I also investigate environmental constraints on sociality, and the impact that workers have on productivity, in the obligate social species L. malachurum. Chapter 1 introduces sweat bees as a study system, and reviews social behaviour within the group. I then provide concise introductions to the study species. In Chapter 2 I show that L. calceatum exhibits latitudinal social polymorphism in the UK, with only bees in the south expressing primitive eusociality. I then describe the social life cycle from continuous field observations, with reference to genetic data. In Chapter 3 in I examine environmental and genetic components of social phenotype in L. calceatum by conducting a field transplant of bees from the north of the UK to the south. Social phenotype is likely to be predominantly determined by fixed genetic differences between social and solitary populations. Chapter 4 examines whether the transition between social and solitary nesting results in saw-tooth size clines in L. calceatum and Halictus rubicundus. Overall, both species exhibit converse-Bergmann clines but not saw-tooth clines. In Chapter 5 I transplant the obligate social sweat bee L. malachurum to the north of the UK, to test whether sociality is constrained by season length. Phenology was considerably delayed such that the life cycle could not be completed. In Chapter 6 I investigate queen quality, productivity and costs of worker production in L. malachurum, by manipulating the number of workers per nest. I show that queens probably incurs costs from producing more workers, and that a possible mechanism is that workers from larger groups may be of lower quality. In Chapter 7 I bring together key findings of the thesis, and comment on future directions.