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Bees & Sunflower

Thursday, March 25, 2021
filed under: Insects

large sunflower head in front of research center
Photo credit: Jarrad Prasifka
        The critical importance of bees to pollination, seedset and, ultimately, sunflower crop yield, was universally accepted 50-60 years ago.  That was a time of exclusively open-pollinated varieties, preceding the development and commercialization of sunflower hybrids and their inherent self-compatibility (self-fertility).
        But even with the omnipresent self-compatible hybrids of recent decades, bees have remained an important component in sunflower crop development.  They still enhance pollination, they still help optimize seedset — and, they still contribute to yield.  The degree of that contribution varies, depending on the specific hybrid being grown and the environmental conditions it experiences.  But the “bee effect” remains significant.
        In early 2019, the Journal of Economic Entomology published an article by entomologists Rachel Mallinger, Jeff Bradshaw, Adam Varenhorst and Jarrad Prasifka* that estimated the economic value of wild, native bees to the U.S. sunflower industry.  Their study encompassed two years (2016 and 2017), three states (North Dakota, South Dakota and Nebraska) and 10 varieties of confection sunflower.  They compared two treatments:  “open to pollinators” and “closed (bagged) to pollinators.”  They also measured pollinator visitation rates and compared per-visit seedset across the various visitor bee species. 
        The difference in average seed mass per ’flower head between “open” and “closed” in the three states for both 2016 and 2017 was striking.  Summarized across all locations and hybrids, insect pollination increased confection yields by 45%, the researchers reported.  Extrapolated across “all-sunflower” commercial production on a regional basis (i.e., the two Dakotas and Nebraska), that percentage yield increase translated into an economic value of more than $40 million; on a national basis, an added value of more than $56 million.
        USDA-ARS Fargo-based research entomologist Jarrad Prasifka refers to those bee-induced yield and dollar impacts as essentially “free money.”  There are no added input costs from wild bee visitation on the farmer’s end, he points out; yet the benefits can be substantial.  “It’s not true for every single hybrid, but it is true for a lot of them,” Prasifka states. 
        He adds that while the Dakota-Nebraska research focused on confection sunflower, work by other researchers suggests native bees’ value to sunflower can also hold true for oil-type hybrids, “making [their overall] value every year to the U.S. sunflower crop likely well beyond $100 million.”
        What plant traits factor into the degree to which bees are attracted to sunflower?  When Mallinger was with the ARS lab in Fargo, she and Prasifka noted several in reporting on their research.  One was the floret length, i.e., the longer the flowers’ corolla, the less attractive the plant was to bees (both wild bees and honey bees).  Then nectar sugar: the more sugar, the more attractive the sunflower.
        That led to further study into the length of the florets themselves.  “Much of bee preference for different sunflower inbreds or hybrids seems to come down to the ease of getting to the nectar — which in turn depends on how deep the sunflower florets are,” Prasifka notes.  “Work with Brent Hulke (ARS sunflower geneticist) and others to map genes that control the size of both florets and seeds suggests we can breed sunflower with floral traits that bees prefer — while still keeping the seed traits that our markets require.”
florets and dime

        Most of the Fargo ARS work has been done with wild, native bees.  But since 2019, Prasifka and his colleagues also have been collaborating on honey bee research with the USDA’s Carl Hayden Bee Research Center in Tucson, Ariz.  “Work there suggests that honey bees’ preferences are similar to the wild bees, meaning that work we have done the past several years targeted at understanding bees’ effects on sunflower grower yields in the Great Plains can also be applied for more-efficient hybrid seed production in California, where honey bees are used,” Prasifka says.
        Why conduct this research in southern Arizona rather than in the Northern Great Plains?  Two reasons.  First, there’s the collaborative expertise that can be tapped into at the ARS Tucson lab.  Secondly, “up here (in the Dakotas), the wild bees dominate what we see in sunflower — even when we have honey bees around our plots,” Prasifka explains.  “Down there (Arizona), it’s such an arid environment that foraging by honey bees on sunflower doesn’t necessarily reflect that they like sunflower better than other plants; but the sunflower does produce a lot of pollen and nectar, and they don’t have a lot of other choices in that arid environment.”  So it is, in essence, an opportunity to “fine tune” the honey bee effect without the presence of native bees clouding the data. — Don Lilleboe
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