Honeybees much less effective than native pollinators for Lepechinia floribunda

How do you measure the effectiveness of pollinators? A common technique is to measure the visit frequency of pollinators, but this might be misleading, say Matias Cristian Baranzelli and colleagues. They argue that pollinators need to visit the flower at the right time of day. Insects like honey-bees may not be achieving much compared to native bees that visit flowers at specific times. They came to this conclusion after examining the effectiveness and efficiency of pollinators visiting Lepechinia floribunda. Displacement of native pollinators by honey-bees may interfere greatly with reproductive biology and ecology in plants.

“[E]ven though the importance of a pollinator depends on its effect on plant fitness, there has been a historical bias to infer the ecological and evolutionary consequences of a plant–pollinator interaction based on its frequency of occurrence,” write the authors. “Although the frequency of visits to flowers provides a useful approximation, it is by no means a definitive assessment of pollinator performance and of its effect on plant fitness.“

The team examined pollinator visits to Lepechinia floribunda, a shrub in the mine family. The flowers are visited by bumble-bees (Bombus spp.), leafcutter bees (Megachile sp.) and honey-bees (Apis mellifera). Baranzelli and colleagues recorded the floral biology of L. floribunda to better understand which interactions mattered most.

They found that nectar volume increased during the day. Critically, the botanists say that the stigmas of the flowers, the parts that receive pollen, also varied during the day. “The first receptive stigmas were observed early in the morning at around 08.00 h but at low frequency (20 %). It was not until 11.00 h that >70 % of the observed flowers had their stigmas open with an angle of >90º that indicated receptiveness. By 13.00 h, >80% of stigmas were receptive.”

The team say they also saw differences in insect visitors. “Average visitation frequency (Vf) was significantly higher for A. mellifera (1.29 ± 0.21 visits per flower h^–1) than for Bombus spp. (0.83 ± 0.13 visits per flower h^–1) or Megachile sp. (0.62 ± 0.08 visits per flower h^–1; F = 4.67; P = 0.0013;). Apis mellifera was the main visitor until 11.00 h, whereas Bombus spp. increased its frequency between 11.00 h and 13.00 h. Megachile sp. showed a low but constant Vf throughout the day. Handling time (Ht) varied between flower visitors (F = 5.85; P = 0.005).”

These differences meant that the honey-bees were often visiting with little benefit for the plant. While there were fewer bumble-bee visits, Baranzelli and colleagues day they were arriving at the right time. “This temporal matching accounted for the higher effectiveness and efficiency of bumble-bees over honey-bees and leafcutter bees. The results demonstrate that the higher visitation frequency of honey-bees was not enough to surpass the higher effectiveness and efficiency of the native bumble-bees.”

“Under present levels of performance of honey-bees, plants would have to produce three times more flowers, or honey-bees would have to double their visitation frequency, to attain a seed production per hour as high as is attained by the native bumble-bees.”

“This comparative study provides new evidence of the factors that play a critical role when estimating pollinator performance. Our results highlight the importance of a more precise distinction among the events during the pollination process and their contribution to final plant fitness. We suggest that combining floral phenology with pollinator activity provides a more realistic model of pollinator performance.”