Nesting aggregation as a determinant of nest parasitism in mason bees

Background/Question/Methods

The evolution of sociality in animals should occur only if it provides a benefit to the organisms such as protection against natural enemies, including parasites. Many bee species are solitary, suggesting they do not receive a net benefit from sociality. Increased attention has been paid to solitary mason bees as managed pollinators, given recent reductions in populations of the European honeybee.  Mason bees are often cultivated in large numbers in communal nest settings for use in agricultural pollination. Mason bees are vulnerable to exploitation by brood parasites, such as kleptoparasitic wasps. Previous studies have found some bee and wasp species to be at increased risk of brood parasitism when nesting in larger groups. A field study was conducted in subalpine meadows at the Rocky Mountain Biological Laboratory in Colorado, USA to assess whether mason bees were at increased risk of brood parasitism as the size of their nesting aggregations increased. Mason bees were allowed to nest in artificial nest boxes and establish natural variation in numbers of nesting individuals within nest boxes. Nest cells constructed by bees were then checked for the presence of kleptoparasite larvae shortly after they were completed.

Results/Conclusions

Overall, nest cells constructed in blocks containing multiple active bees were significantly more likely to be oviposited in by brood parasites compared to cells constructed in blocks with fewer active nesting bees. This suggests that gathering in large aggregations for nesting can negatively affect fitness of mason bees, and that the risk of parasitism may be a factor in explaining solitary behaviour in these species. These results also have implications for the management of mason bees as agricultural pollinators, as cultivating them in large groups could lead to increased vulnerability to brood parasitism, which could in turn reduce reproductive output and therefore the sustainability of cultivated bee populations.