Effects of social organization and resource availability on brood parasitism in the facultatively social nocturnal bee Megalopta genalis

Understanding the ecological benefits that may select for the evolution of living in groups rather than solitarily is key to understanding the evolution of social cooperation. Defense against natural enemies, such as parasites and predators, is generally acknowledged to be such a benefit, but most studies focus only on predators. Among the bees, parasitism is a major source of brood mortality. Here we use the nocturnal, Neotropical, facultatively social bee Megalopta genalis (Halictidae) to directly compare parasitism rates between social and solitary nests at the same site during times of high and low resource availability. We focus on four parasites: one that eats some pollen but does not kill host offspring [the fly Fiebrigella sp (Chloropidae)] and three that kill host offspring (the beetle Macrosiagon gracilis (Rhipiphoridae), the wasp Lophostigma cincta (Mutillidae), and the congeneric bee M. Noctoraptor byroni). We found no difference in parasitism between social and solitary nests for the four parasites tested. Across all nests, 25–31% of brood cells were parasitized by Lophostigma sp., with parasitism increasing from the dry to wet season, although this seasonal effect was significant only for social nests. Only ~ 4% of brood cells were parasitized by one of the other three parasites, and M. byroni was only found in social host nests. Our data reveal that Megalopta have low rates of parasitism overall, and that this is not a function of social organization or resource availability. Limited nocturnal foraging time and foraging offset from the activity patterns of diurnal parasites may instead play a larger role in keeping parasitism rates low.