Annual variation in survival and reproduction of the primitively eusocial sweat bee Halictus ligatus (Hymenoptera: Halictidae)

We studied a nesting aggregation of the primitively eusocial sweat bee Halictus ligatus near Victoria in southern Ontario during the summers of 1984, 1990, and 1991. Differences in local weather patterns from year to year had marked effects on bee demography and behaviour, belying previous conclusions about "typical" social organization in this aggregation. In 1990, comparatively cool, rainy weather resulted in high nest-failure and low brood-survival rates, while in 1984 and 1991, relatively dry, warm weather had the opposite effect. In 1984 and 1990, spring nest initiation was synchronous and the emergence periods of the first (worker) and second (reproductive) broods were temporally distinct. In 1991, exceedingly warm spring weather caused asynchrony in the timing of nest initiation, accelerated brood and colony development, and continuous brood production. In 1984 and 1990, a few males were produced in the first brood but most were produced in the second brood several weeks later. In 1991, continuous brood production meant that production of males represented the transition between production of workers and of gynes (second-brood females). Patterns of demographic and social variation exhibited by H. ligatus at Victoria parallel those observed on a continent-wide geographic scale. This suggests that primitively eusocial sweat bees maintain a variety of reproductive options, adjusting their social behaviour in response to local environmental conditions.