Evidence of negative impacts on native bees from the widespread introduction of honeybees (Apis mellifera) has been a subject of much study. Direct evidence of competition, however, has been difficult to demonstrate. Although competition occurs only when a resource is limiting, two previous studies measuring nectar consumption by native bees and honeybees found little evidence that nectar was a limiting resource. In this study, conducted in tallgrass prairie restorations in northeastern Illinois and northwestern Indiana, we looked for nectar limitation among honeybees and native bees by measuring nectar volumes in flowers of bee balm (Monarda fistulosa) a common species visited by both groups of bees. In each of the three sites with an introduced honeybee hive, we measured nectar volume in flowers from four inflorescences on separate ramets every two hours during the active foraging period on one day in early August 2015. During the early morning before the bees were active, we secured netting with different mesh sizes over individual inflorescences to exclude bumblebees (Bombus spp.; large [6.35 mm] mesh), honeybees and bumblebees (medium [3.18 mm] mesh) or all flower visitors (small [bridal veil] mesh). The fourth inflorescence measured in each site had no netting. We anticipated that nectar would continue to be available throughout the day as found in the two previous studies, and that as a result competition for nectar as a limited resource would not be found.
Results/Conclusions
Surprisingly, two distinct patterns of nectar consumption emerged. Flowers were found to have either no measurable nectar volume or to have measurable nectar. No nectar was found in the flowers with no netting and in flowers with large mesh netting, while flowers with medium and small mesh netting had measurable nectar quantities. The treatments with no measurable nectar were the flowers to which honeybees had access. Unfortunately, honeybees could not be excluded without excluding larger native species, so nectar consumption by honeybees alone could not be quantified. The total depletion of nectar volumes through consumption, however, is evidence for competition for nectar in Monarda fistulosa flowers.