March 31, 2005: Many farmers who grow fruits and
vegetables arrange to have honeybee hives situated near their fields
and orchards in order to ensure adequate pollination. But new evidence
suggests they should take steps to protect and encourage native pollinators
Dr. Claire Kremen, assistant professor of ecology and evolutionary
biology at Princeton University, led a study of the role of native
bees in pollination of watermelons, cherry tomatoes, and hybrid
sunflowers in the Central Valley of California. Her research team
found that native bees provide significant pollination services
for these crops—in some cases enough to make honeybees unnecessary.
They also found that the proximity of natural areas was a significant
factor in native pollinator populations on farms and that organic
farms tended to have more vigorous native pollinator populations
than conventional farms.
There are some 4,000 native bee species in North America, says
Kremen, and as many as 50 to 60 species may be found in a given
agroecosystem. Ongoing threats to the European domesticated honey
bee, including parasitic mites and hybridization with aggressive
African bee strains, have caused a 50 percent decline in U.S. honey
bee colonies since 1950 and a 70 percent decline in feral honey
bees, according to Kremen. These factors are making commercial pollination
services more expensive—and thus the protection of native
pollinators more important.
Like all cucurbits, watermelons have separate male and female flowers
and are dependent on insects for pollination. Individual female
flowers open for a single day and must receive 500 to 1000 pollen
grains to produce a marketable melon. Kremen's team found that native
bees were more effective than honey bees in pollinating watermelons,
in part because they forage earlier in the day and transfer more
pollen per visit.
The researchers found 23 native bee species visiting watermelon
plants in Yolo County. The five most important were the yellow-faced
bumble bee (Bombus vosnesenskii), the California bumble
bee (Bombus californicus), the squash bee (Peponapis pruinosa),
long-horned bees (Melissodes species) and sweat bees (Halictus
Tomato plants are self-fertile, but the flowers must be vibrated
for their pollen to be released. Usually, the wind is enough to
vibrate tomato plants, but insect visits can help significantly.
Honey bees don’t normally visit tomato flowers because the
flowers have no nectar and honey bees are unable to vibrate the
tomato flower to release the pollen. Most tomato pollinating is
performed by bumble bees and other native species.
Kremen's team demonstrated that cherry tomatoes visited by native
bees produced larger and more numerous fruits than those pollinated
by wind alone. Five native bee species were found visiting cherry
tomatoes on organic farms in Yolo County, the most important being
the yellow-faced bumble bee (Bombus vosnesenskii), the
California bumble bee (Bombus californicus) and the mud
bee (Anthophora urbana).
Bee pollination is essential to the production of hybrid sunflower
seeds. While growers typically use honey bees for this purpose,
Kremen's group found that native bees are also important pollinators.
At least 29 native bee species were found to visit sunflowers in
Yolo County. The most important were long-horned bees (Melissodes
spp.), sunflower bees (Diadasia and Svastra spp.),
the yellow-faced bumble bee (Bombus vosnesenskii) and the
sweat bee (Halictus ligatus).
Most interestingly, the researchers demonstrated that interacting
populations of honey bees and native bees resulted in the best pollination
rates, evidently because the different types of bees skirmish with
one another, causing more movement between the interspersed rows
of male-sterile and male-fertile plants.
Organic makes a difference
Monitoring native bee populations on organic farms close to wild
habitat, organic farms distant from wild habitat, and conventional
farms distant from wild habitat (conventional farms tend not to
located with close proximity to wild habitat), Kremen's research
group found that 80 percent of organic farms close to wild habitat
could rely entirely on native pollinators for pollination services.
By contrast, just 50 percent of organic farms located distant from
wild habitat could rely on native pollinators alone, and none of
the conventional farms had sufficient native pollinators for pollination
needs. Overall, native bees provided 28 percent of pollination on
conventional farms and 60 percent on organic farms.
Enhancing pollinator habitat
Farmers can improve native pollinator populations on their farms
by supporting natural area protection and restoration efforts in
their communities; leaving field and road borders untilled to provide
habitat for ground-nesting bees; eliminating or reducing insecticide
use; and planting or preserving native flowering plants in pastures,
hedgerows, and other areas on their farms.
Kremen has collaborated with the Xerces Society for Invertebrate
Conservation, a non-profit group based in Portland, Ore., to produce
three fact sheets summarizing her findings and suggesting ways for
farmers to encourage native pollinator populations. The fact sheets
can be downloaded free of charge from the Xerces Society website
The Xerces Society has also published a 34-page booklet, Farming
for Bees: Guidelines for Providing Native Bee Habitat on Farms
and 145-page book, the Pollinator Conservation Handbook.
Laura Sayre is senior writer for NewFarm.org.