Trapping studies reveal phenology and reproductive behaviour in two solitary sweat bees (Hymenoptera: Halictidae)

A transition from univoltine to bivoltine life histories, potentially due to the lengthening of flight seasons, is thought to have preceded the evolutionary transition from solitary to eusocial behaviour in halictid bees. Most behavioral studies which allow us to map these demographic and social transitions on phylogenies have focused on eusocial species, leaving a gap in behavioral data regarding solitary halictid bees. This lack of behavioral trait data is due to the great difficulty in locating the nests of solitary bees for study. However, long-term trapping studies are an effective method providing insights into the demography and social status of halictid bees. We used pan-trapped specimens to investigate flight phenology and solitary reproduction Lasioglossum leucozonium (Schrank) and L. zonulum (Smith), from the socially and demographically labile subgenus Leuchalictus . We found L. leucozonium to be univoltine and solitary across its range, regardless of the length of flight season. In contrast, L. zonulum is phenologically flexible, being univoltine in the shorter flight seasons of Alberta, but bivoltine in southern Ontario. In both L. leucozonium and univoltine L. zonulum , higher than expected numbers of old, worn females with declining ovarian development foraged late into the season, suggesting that an indeterminate breeding strategy represents an ancestral condition not found in eusocial species. In bivoltine L. zonulum , the large size and higher fecundity of Brood 1 offspring suggest that after evolutionary reversion from eusocial to solitary behaviour, selection for worker-sized daughters may be reversed to selection for larger body sizes. The demographic plasticity demonstrated by L. zonulum may hint that partial bivoltinism is more widespread than currently documented. Future studies aimed at clarifying the demographic and social traits of more halictids will pave the way for comparative studies and eventual confidence in tracing how behavioral traits have changed and evolved in halictid bees.