RESEARCH UPDATE
It pays to know (and protect) your pollinators
California research documents the value of native pollinators in watermelon, cherry tomato, and hybrid sunflower production

By Laura Sayre

A California native, the yellow-faced bumble bee acts as a pollinator for watermelon, cherry tomatos and hybrid sunflowers. Photo by Jeff Adams.

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.

Watermelons

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 species).

Cherry tomatoes

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).

Hybrid sunflowers

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 at .

The Xerces Society has also published a 34-page booklet, and 145-page book, the .

Laura Sayre is senior writer for NewFarm.org.