Resource requirements of wild bees in The Colorado Rocky Mountains and the Tucson Sky Islands

Background/Question/Methods

Wild bee populations have been decreasing on a global scale. Yet, little work has been done to understand which specific factors are important for bee population growth and survival.Floral food resources, as well as the availability of nesting habitat, are two factors that are likely to play a critical role for bee populations. We set out to identify how these two factors influence the nesting success of Megachilid bees at two geographic locations in western USA. To address this, we deployed a series of artificial nest boxes at 11 sites in the Colorado Rocky Mountains in 2012 (120 boxes), and at 6 sites in the Tucson Sky Islands in AZ in 2013 (60 boxes).  During the activity periods of Megachilid bees we measured the abundance and diversity of floral resources at each site. At these same sites, we also quantified the quality and abundance of habitat resources using a novel protocol. We hypothesized that both floral resources and nesting habitat will play an important role in nesting success, but the magnitude of the importance of each will depend on abundance and quality at each site.

Results/Conclusions

We find variation in the abundance of floral resources, and abundance and quality of nesting habitat resources across sites at both geographic locations. Sites with higher nesting quality and abundance were associated with a greater number of bees nesting in the artificial houses. Regarding floral resource, we find that despite the variation of floral abundance across sites, there was no relationship with nesting success. As a result, we find that bee nesting success in the experimental bee boxes is based on the quality and abundance of nesting resources more than floral abundance. These findings suggest that despite the importance of floral resources for wild bees, nesting sites may be more of a limiting factor in determining bee abundance. This study highlights the importance of addressing specific key factors related to wild bee nesting and population size.