Can Bees Build Soybean Yields?

A self-pollinating crop like soybeans shouldn’t have a need for bees. But recent research into the little-understood relationships between the two indicates there could be big bean yield benefits from bees. Creating more bee-friendly habitats could prove to be a worthwhile goal for soybean growers.

At Iowa State University (ISU), Matt O’Neal’s lab started out looking for conservation practices that would encourage beneficial insects such as the syrphid fly larvae, which eat soybean aphids. “We were approached by the industry to look further at bees in soybeans,” recounts the entomologist. “It’s remarkable. We’ve seen more bees than I initially thought were there. During a single summer we collected close to 2,000 pollinators and some 20 different bee species.

“So can we prove they are there for the soybeans rather than the bee traps?” he asks. His team checked the bees and found 20-25% carried soybean pollen.

“They are going to the flowers, but we don’t know yet what that means for the plant,” he says. “The challenge is to see whether they affect yield. There is some evidence in scientific literature that they may improve yields.”

Three examples of earlier research include:

• A short-term Canadian study found bees’ presence was associated with much higher yields in food-grade soybeans.
• Australian researchers demonstrated yield increases of 10-40% in honey bee-pollinated soybeans, compared to self-pollinated beans.
• In 2005, a Brazilian research project compared soybean seed production with and without honey bee colonies by raising plants in cages, and reported 50% higher yields when bees were present.

While all three showed yield improvements, they involved honey bees rather than the native bees O’Neal’s research found.  Among the thousands of native bee species, some are more effective pollinators of specific plants than honeybees. It’s a challenge to link specific bees’ activities to the soybean plants and then to yield.

ISU’s Reid Palmer, a research geneticist who studies plant and insect pollinator attraction and reward traits, suggests pod set is a possible explanation for the yield bump. “Soybeans naturally drop a lot of flowers, and not from lack of fertilization,” he says. “Plants usually produce many more flowers than develop into pods, so if you can reduce flower drop, you should increase yield.”

O’Neal is part of an interdisciplinary team conducting field trials to see what happens when small strips of prairie are incorporated into row-crop landscapes. (See http://bit.ly/CSDstrips for more information.) “Integrating a little bit of prairie can produce big changes in soil erosion and nutrient loss,” he explains. “That same prairie could also be habitat for beneficial insects like bees.”

CRP land, roadsides, terraces or any wasteland might be suited to such a conservation “triple stack.” Native bees in particular might benefit from forage plants and undisturbed ground for nesting. Farmers with CRP acres can check with their local FSA office for details on the CP42 Pollinator Habitat practice, which can qualify CRP land for higher payments.

One challenge – the issue of which plants to incorporate in native planting – is being resolved by multiple research efforts around the country. O’Neal has found good results in Iowa with two non-invasive candidates – meadow zizia and cup plant – without seeing an increase in weeds. He’s now determining how much bee habitats benefits soybean plants.

“It’s all connected,” he concludes. “If you think about these issues individually, you miss the opportunity for multiple benefits.”

Best plants for pollinators

Researchers are developing improved lists of which plants bees like best, and organized region-specific list that can be accessed through the Xerces Society’s Pollinator Resource Center: http://bit.ly/S56MUp

Other sources include:

USDA – NRCS

Michigan State University

Native Pollinators in Agriculture

Pollinator Partnership