Variable worker behaviour in the weakly eusocial sweat bee, Halictus sexcinctus Fabricius

Studies of eusocial halictines suggest that workers have many reproductive options, including sterile altruism in the maternal nest, combined helping and personal reproduction, and diapause and spring nest founding. How and when workers exercise these various options influences the strength of colony social organization. Halictus sexcinctus exhibits highly polymorphic social behaviour, with solitary colonies in central Europe and both eusocial and communal colonies in southern Greece. Indirect evidence suggests that some worker-brood females are actually gynes. A distinctly bimodal size distribution among foundresses in 1998, the lower size peak being close to the modal body size of workers from 1997, suggests that large worker-brood females overwinter and return to the aggregation as eusocial foundresses. Other first-brood females remain in the maternal nest as workers, although few can be classified as classical, sterile altruists. Only 17% of older, healthy workers are sterile (i.e. had ovarian development scores ≤ 0.1), whereas about 83% are reproductive, exhibiting at least one 1/4-developed oocyte. About 57% of older workers have at least one fully or 3/4 developed oocyte, signifying that they are ready or almost ready to lay. Sterile workers exhibit greater total wear (combined mandibular and wing wear) scores than reproductive workers, suggesting that they are older, have higher activity rates, or both.