Nest architecture and brood mortality in four species of sweat bee (Hymenoptera; Halictidae) from Cape Breton Island

Abstract

The nest architecture of four species of the bee family Halictidae from Cape Breton Island, Nova Scotia, was studied. Augochlorella striata and Lasioglossum (Evylaeus) comagenense constructed their brood cells in a cluster surrounded by a cavity. In the case of A. striata, the cavity-forming habit increased the rate at which the soil of the brood cell cluster dried out, relative to the surrounding soil. The year of the study was unusually dry and in midsummer the moisture content of the soil was extremely low. This weather pattern seemed to result in some mortality of A. striata brood due to dehydration, and prevented foundresses from constructing brood cells. The two other species, Lasioglossum (Evylaeus) cinctipes and Lasioglossum (Dialictus) laevissimum, did not construct brood cell clusters. All four species had shallower nests than other species with similar nest architectures for which published data are available. All species except L. laevissimum nested in association with stones at the ground surface. It was shown that brood cells of nests situated close to rocks and stones were likely to experience increased temperature, presumably resulting in a faster developmental rate for immatures. This may be important in the Cape Breton environment, which is clearly marginal for social sweat bees. Both A. striata and L. comagenense exhibited very low mortality rates in developing immatures (1 and 7%, respectively).