Bumble Bees Tested as Replacement Pollinator for Honey Bees

A recent study at Oregon State University has shown that native bumble bee species have consistently high pollination and seed production levels in red clover.
 

The bumble bee pollination findings offer promise for the use of bumble bee crop pollinators as an alternative to European honey bees, whose populations have recently declined in many areas of the United States.

Red clover, which is grown for forage and as a rotation crop to improve soil, is raised for seed in western Oregon's Willamette Valley. It will not produce seed without pollination, so growers typically place two to five European honey bee hives on each hectare. 

Search for Alternative Pollinators

Bee diseases, mites, and colony collapse disorder have recently limited availability and resulted in higher costs for hive rentals. Given these changes, an alternative pollinator for red clover seed crops is needed.

Worldwide there are over 200 species of bumble bees; some of whom are known to pollinate red clover. While commercially reared bumble bee species are available to growers elsewhere, they are considered exotic species in Oregon and cannot be introduced into the state. This leaves Oregon growers dependent on naturally occurring populations of bumble bees as pollinators. However, there is currently no information on the pollination efficiency of native bumble bee species.

Scientific Evaluation of Blooms and Seed Set

Through funding from the Clover Commission, scientists at Oregon State University investigated native bumble bees in commercial fields of red clover seed crops in the Polk County region of the Willamette Valley. Prior to bloom, researchers covered plants with mesh-screened cages. European honey bee hives were placed in some cages and nests of B. vosnesenskii, a native Oregon bumble bee,in others. Some cages were also left vacant. After bloom, seed yield and seed set were compared amongst the different cages. Seed set was also evaluated in four different fields without cages to assess the efficiency of existing bee pollinators. In addition to analyzing seed set, researchers assessed the diversity and abundance of native bumble bees through visual observations of foragers on red clover flowers and through trapping bumble bees in blue vane traps. 

Pollination Differences between Bumble Bees and Honey Bees

While there were no differences in seed yield or average seed set in cages with bumble bees compared to honey bees, the study revealed:

• Variability across cages was lower with bumble bees indicating that bumble bee pollination is more uniform than pollination by European honey bees.
• The abundance of bumble bee peaked during mid-to-late bloom.
• They recorded six species of bumble bees gathering pollen from red clover flowers. Of these, more than 92% consisted of B. vosnesenskii, indicating that it is the key pollinator in Oregon.
• 25 more species of native solitary bees, belonging to 12 genera and five families, were collected in the bee traps.

Research Findings

The study has not only documented a great diversity of native bees in synchrony with red clover bloom, but it has also found that seed set was uniform and high across four fields. Under current pollinator regimes, researchers believe red clover seed production is close to its maximum in Oregon.

Solutions for Pollination

"To sustain these high yields in Oregon, we must

• Conserve the habitat of bees,
• Use pesticides judiciously and
• Provide floral resources prior to red clover bloom

Oregon State University entomologist Sujaya Rao, one of the researchers on the study, concluded,  "Globally, where red clover seed is produced, similar studies are needed. If seed set is found to be well below the maximum, appropriate alternative options such as augmentation with commercial bumble bees could be considered."

Research is ongoing at Oregon State University to determine whether high yields can be achieved by native pollinators alone. If so, European honey bee hive rentals would not be required, and this could lead to more economic red clover seed production in Oregon.

The Crop Science Society of America (CSSA), founded in 1955, is an international scientific society comprised of 6,000+ members with its headquarters in Madison, WI. Members advance the discipline of crop science by acquiring and disseminating information about crop breeding and genetics; crop physiology; crop ecology, management, and quality; seed physiology, production, and technology; turfgrass science; forage and grazinglands; genomics, molecular genetics, and biotechnology; and biomedical and enhanced plants.

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