Red clover, which is grown for forage and as a rotation crop to improve soil, is raised for seed in western Oregon’s Willamette V alley. It will not produce seed without pollination, so growers typically place two to five European honey bee hives on each hectare.
However, bee diseases, mites, and colony collapse disorder have limited their availability and resulted in higher costs for hive rentals. Hence, an alternative pollinator for red clover seed crops is needed.
Worldwide there are more than 200 species of bumble bees, and some of these are known to pollinate red clover, but there is no information on the efficiency of the species that are present in Oregon. While commercially reared bumble bee species are available to growers elsewhere, these exotic bees cannot be introduced into Oregon, and so Oregon growers depend on naturally occurring populations.
In the November–December issue of Crop Science, scientists at Oregon State University report on their investigation of native bumble bees in commercial fields of red clover seed crops in Polk County in the Willamette Valley. Prior to bloom, they covered plants with mesh-screened cages and placed European honey bee hives in some cages and nests of the native Bombus vosnesenskii in others. They also left some cages vacant. When bloom ended, they removed the cages and compared seed yield and seed set. They also assessed the diversity and abundance of native bumble bees through visual observations of foragers on red clover flowers and by trapping them in blue vane traps. Seed set was compared in four different fields to assess the efficiency of existing bee pollinators.
The study revealed that while there were no differences in seed yield or average seed set in cages with bumble bees compared with cages with honey bees, variability across cages was lower with bumble bees, indicating that bumble bee pollination is more uniform than pollination by European honey bees. The researchers observed that the abundance of bumble bees 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. The same six native bumble bee species were caught in the traps, which also caught 25 species of native solitary bees belonging to 12 genera and five families, documenting a great diversity of native bees in synchrony with red clover bloom. The study also found that seed set was uniform and high across four fields and that under current pollinator regimes, red clover seed production is close to its maximum in Oregon.
“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,” says Oregon State University entomologist Sujaya Rao, one of the researchers on the study. “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.
Adapted from S. Rao and W.P. Stephen. 2009. Bumble bee pollinators in red clover seed production. Crop Sci. 49:2207–2214. View the full article online at http://crop.scijournals.org/content/vol49/issue6