Impacts of landscape structure and pesticides on wild bees in southern Wisconsin

Background/Question/Methods

Native, wild bees provide the important ecosystem service of pollination to both crop plants and wild plants. Current concern over a decline in pollinators and pollination services extends to both native, wild bees as well as to managed honey bees. Wild bees are threatened by agricultural intensification, including pesticides, and the loss of habitats that provide foraging and nesting resources. The objectives of this research were to examine how landscape structure and pesticide use affect wild bee abundance and diversity in a pollinator-dependent crop. Thirty apple orchards differing in landscape composition were chosen as study sites. Landscape metrics at each site were determined with land-cover data. The abundance and diversity of wild bees were determined through bee bowl sampling during the spring and summer of 2010. The cumulative toxicity of all chemical sprays applied in the spring and summer of 2010 at each orchard was calculated using growers’ spray records. We hypothesized that the abundance of wild bees would be lower at sites surrounded by intensive agriculture and at sites with high toxicity scores. We also hypothesized that the diversity of wild bees would be higher at sites with high habitat diversity in the surrounding landscape.

Results/Conclusions

The abundance of wild bees collected during bloom showed a weak, positive relationship to habitat diversity at a 1K radius surrounding each site (R²=0.13, P=0.05). Species richness of wild bees was also significantly, positively related to habitat diversity (R²=0.19, P=0.02). Mantel tests revealed a significant relationship between habitat diversity and wild bee community composition (P=0.04). The toxicity of pesticides applied during bloom was relatively low and had no significant effects on bee abundance or diversity.  Preliminary results additionally show that wild bee abundance in the sampling area was significantly affected by the local density of apple blossoms. These results suggest that abundance of wild bees is sensitive to local factors, such as the density of floral display, in addition to landscape structure, while the diversity of wild bees is most closely related to habitat diversity.  Habitat diversity may increase the availability of nesting and foraging resources for different bee species, including specialists, and reduce interspecific competition.