Two bees, or not two bees?
Wild bees make honeybees 'skittish,' new study finds, boosting both pollination efficiency and crop yields
| 30 August 2006
When honeybees interact with wild native bees, they are up to five times more efficient in pollinating sunflowers than when native bees are not present, according to a new study by a pair of UC researchers at the Berkeley and Davis campuses.
Coming at a time when populations of honeybees - a species imported into the Americas centuries ago - have been decimated by parasitic mites, the findings suggest that protecting wild native bees and their habitat could play a crucial role in ensuring adequate pollination for a host of important crops.
(Sarah Greenleaf photo)
"Up until now, we've thought that honeybees alone were doing most of the pollination," says Sarah Greenleaf, a postdoctoral researcher in plant pathology at UC Davis and the study's lead author. "But now we know that a lot of honey-bee pollination happens because of their interaction with wild native bees. This means that wild bees are much, much more important than we previously thought."
The study is being published this week in the online early edition of the Proceedings of the National Academy of Sciences and will appear as the cover story in the journal's Sept. 12 print issue.
Working with Berkeley conservation biologist Claire Kremen, an assistant professor in the College of Natural Resources, Greenleaf observed the behavior of honeybees from managed hives and wild native bees in sunflower fields during two growing seasons. The sunflowers were being grown for hybrid-seed production on 16 farms in Yolo and Solano counties.
In fields where wild bees were rare, a single visit by a honeybee produced an average of three seeds. But as wild-bee numbers increased, so did the number of seeds produced per honeybee visit, up to an average of 15 seeds per visit. This was the case when either the richness of the species mix of wild bees increased or the absolute number of wild bees increased.
When bagging it means working hard
To find out how many of the approximately 1,500 flowers that comprise a sunflower head a bee pollinated, Greenleaf would cover each immature sunflower head in the plot she was working in with a mesh bag. When the flowers were open and ready for pollination, she would remove the bag and stand watch until a honeybee landed and went to work. As soon as the bee flew off, she'd re-bag the flower head, returning a month later to count the seeds that had been produced.
When Kremen and Greenleaf followed the behavior of their tiny subjects, they discovered the reason for the boost in pollination: Like the captain of a plane switching out of autopilot when she spots a craft nearby, a honeybee alters its flight pattern after meeting up with a wild bee on a sunflower head.
Many plants - including sunflowers used for hybrid-seed production - produce two kinds of flowers: pollen-bearing male flowers and nectar-bearing female flowers. In hybrid-sunflower-seed production, rows of one cultivar that bears only male flowers are interspersed among rows of another cultivar that bears only female flowers.
Because foraging honeybees are specialized workers - some typically collecting pollen and others nectar - anything that causes them to alter their foraging behavior improves the likelihood that they will move between different kinds of flowers, resulting in pollen being transferred to the place where it's needed: a female flower part.
In the sunflower fields, the researchers found that when pollen-gathering honeybees met up with other honeybees, they generally stuck to their regular routine, foraging down the row from one male plant to the next. A mere 7 percent shifted rows and wound up on a female flower after such an encounter. But 20 percent of the honeybees that encountered a wild bee buzzed off to another row and alighted on female flowers.
"Growers can throw more and more honeybees out there, but they're not going to get more pollination if the bees visit only one of the cultivars," Kremen says." Wild bees make the honeybees more skittish, so they move more frequently between the different cultivars. Each time they move, there's a possibility of them transporting the pollen between the rows."
Since the 1980s, when two species of mites that parasitize honeybees were inadvertently introduced into the U.S., populations of honeybees living in the wild have all but disappeared, and the number of managed hives has plummeted from 4 million to 2.4 million. Along with honeybee declines, populations of wild bees are also dropping, Greenleaf says. Habitat loss and "unfriendly farming practices" have both taken a toll.
For crops such as the hybrid sunflower, which was already receiving inadequate pollination, declining bee populations may be reducing yields, Greenleaf says.
"We've only recently started to document the services that nature provides us," Kremen says. "In this case, the agents providing services are both commercially available honeybees and the wild bees living in the natural habitat surrounding the farm. The surprising finding here is how important the interaction between honeybees and other species is."
Greenleaf and Kremen say they would expect to see similar pollination gains in a range of other crops, including hybrid seed crops (such as onions and cotton) that are produced in the same way as hybrid sunflower seeds; tree crops that require cross-pollination between two cultivars, including almonds, cherries, and most apples; and crops that bear separate male and female flowers, such as squashes, watermelons, and kiwis.
Conserving patches of natural habitat for native bees in agricultural areas could help maintain their populations and provide better pollination for crops, Kremen says. Both in this study and in previous studies she has conducted in various crops, the closer her study plots were to native habitat, the greater the diversity and abundance of native bees.
"Given that we don't have enough honeybees," Greenleaf says, "it's really great that there's a way to make the ones that are left better pollinators."
Kremen and Greenleaf conducted their research from 2001 to 2003 when Kremen was on the faculty of Princeton University and Greenleaf was a doctoral student there. Kremen joined the Berkeley faculty in 2005.