Bees' use of floral resources in habitats dominated by the exotic plant Centaurea stoebe

Background/Question/Methods

Spotted knapweed (Centaurea stoebe) is an exotic invasive plant that has recently become the target of a biological control program in Michigan. While spotted knapweed is seen by many as a harmful invader, it does represent a significant floral resource throughout the state and is used by managed and native pollinators alike. Because biological control has the potential to change floral resource availability on a landscape scale, it is important to understand how spotted knapweed is being used by pollinators, and whether other flowering plant species have the potential to replace knapweed's role as a source of pollen and nectar. Spotted knapweed-dominated habitats could either A) support similar bee communities as diverse flowering plant communities, or B) support bee communities that differ in abundance or diversity from those supported by diverse flowering plant communities.

To test these hypotheses, we located sites that have fields that are dominated by spotted knapweed that are adjacent to fields containing a diverse assemblage of flowering plants. We estimated the floral abundance each site type provided at different times in the growing season. We then quantified the floral use of the bee community using walking transects. These data were collected over the summer of 2012. 

Results/Conclusions

In mid-summer, spotted knapweed had more visitations by both native bees and honey bees than any other plant species. This finding was consistent in both knapweed dominated and diverse habitat types, which indicates that knapweed is a highly attractive plant to many bee species and was not only being used because of its abundance. However, by late summer knapweed had nearly stopped blooming, and habitats containing a diversity of flowering plants supported significantly more bees. Several Solidago spp. were especially important during this period. This research shows that while spotted knapweed is an important floral resource in degraded habitats, it cannot take the place of a diverse flowering plant community when it comes to providing nectar and pollen to bees throughout the entire season. If there is a decline of knapweed populations in the future, it will be important that flowering plant species are able to colonize knapweed-dominated sites to ensure that these habitats continue to provide floral resources to managed and native bee communities.