Data from: Phenological, but not social, variation in response to climate differences in a eusocial sweat bee, Halictus ligatus, nesting in southern Ontario

Studies of annual variation within populations and geographic variation among populations suggest that more stringent environmental conditions result in stronger reproductive skew favouring queens, whereas moderate conditions favour increasing worker reproduction. Our first objective was to compare the phenology and colony development of H. ligatus nesting in St. Catharines, Ontario, Canada to a previously studied aggregation about 90 km in Victoria, Ontario. Although the two sites are separated by only about 90 km, St. Catharines has considerably shorter winters and longer summers. Comparisons between St. Catharines in 2006 and Victoria in the 1980s and 1990s incorporate both geographic differences in climate and temporal differences due to climate change. We predicted that St. Catharines foundress queens should emerge from hibernation and initiate nests earlier in spring, giving them time to produce more workers. Since queens have difficulty suppressing ! worker reproduction in larger colonies, we also predicted higher rates of worker ovarian development in St. Catharines. A second objective of our study was to assess how well pan trap collections capture important information about demographic and social parameters important in assessing social variability in sweat bees. We excavated 65 H. ligatus nests, comparing their contents to 713 specimens collected in pan traps from April – September 2006. The two sampling methods produced similar results, suggesting that pan trapping is a good alternative when nest excavations are not possible. As predicted, nests were initiated about a month earlier in St. Catharines than in Victoria, but contrary to prediction, colonies were similar in size, and St. Catharines workers were just as likely to have developed ovaries as Victoria workers. This may have been because early queen mortality in the longer nest cycle of St. Catharines bees enhanced opportunities for worker reproduction ! despite their smaller numbers. About 60% of H. ligatus workers could be classified as non-reproductive altruists. Among reproductive workers, larger individuals tended to have higher ovarian scores. Additional novel features of H. ligatus sociobiology in St. Catharines included evidence that queens can initiate new nests following the loss of their first brood, that worker and gyne production may occasionally overlap, and the observation of high rates of independent nest founding by worker-sized females, suggesting that many worker-brood females overwinter in this population. Overall, the distinctly warmer climate of St. Catharines compared to Victoria led to earlier nest initiation and lengthening of the flight season, but not to changes in colony social organisation and queen-worker reproductive skew.