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Role of Pollinators in Confection ’Flowers

Wednesday, March 29, 2017
filed under: Insects

   Fifty years ago, at a time when sunflower producers’ planting seed options consisted solely of open-pollinated varieties, the importance of bees was unquestioned.  An absence of pollinators translated into little or no seedset and minimal yield.  The development and commercialization of hybrid varieties in the 1970s changed all that in a huge way, of course, with the inherent self-compatibility (self-fertility) that they brought to the table.
       But bees have continued to benefit sunflower production during the ensuing decades.  Seedset, oil percentage and final seed yield are, for most hybrids, enhanced by the presence of natural pollinators.  The degree of those benefits varies, depending on the individual hybrid and the environmental conditions under which it is grown.  But it is there — though often overlooked.
       Recent studies by the USDA Agricultural Research Service at Fargo, N.D., have been shedding more light on the role of bees in modern-day sunflower production — and specifically, that of confection sunflower.  ARS research entomologist Jarrad Prasifka began working on the relationship between sunflower and pollinators in 2013.  Since 2015, post-doctoral research entomologist Rachel Mallinger has worked with Prasifka to continue and expand the project.  Cooperators have included Adam Varenhorst at South Dakota State University and Jeff Bradshaw with the University of Nebraska’s Panhandle Research and Extension Center at Scottsbluff.
       At the 2016 National Sunflower Association Research Forum, Mallinger and Prasifka reported on their findings regarding bee/sunflower interactions, specifically regarding sunflower plant traits that attract bees and sunflower’s dependence upon bees.  More recently, at the 2017 NSA Research Forum, Mallinger discussed their research on the benefits of insect pollination for confection sunflower.  That work encompassed comparisons across three states (North Dakota, South Dakota and Nebraska) and multiple hybrids.
       What plant traits factor into the degree to which bees are attracted to sunflower?  There are several, Mallinger and Prasifka point out.  In evaluating 20 inbred lines of oilseed sunflower (10 A-lines and 10 B-lines) in 2015, they found that the longer the floret’s corolla length, the less attractive it was to bees (both wild bees and honey bees).  On the flip side, the higher the nectar sugar amount, the greater the attraction to bees.  Though the ratio of different sugars (sucrose/fructose/glucose) varied among lines, additional study is needed to establish how sugar composition impacts bee visitation.
       As to the benefits of insect pollination for confection sunflower, the ARS entomologists point out that hybrid self-compatibility is not a 100% “all-or-nothing” condition.  There is variation in the level of self-fertility across plant genotypes (e.g., variation in morphological traits), and there is variation depending upon the environment (e.g., geographic location and growing conditions).
       In 2016, the researchers evaluated pollinator-dependency for 10 commercial confection hybrids at sites in the three above-noted states.  They compared insect-exclusion (bagged heads, thus relying strictly on a hybrid’s self-fertility) with open-pollinated treatments (bee visitation unblocked, thus receiving pollinator benefits).  Here’s a summary of their findings:
  • In the North Dakota trials, there was a 22% increase overall in seed weight per flower head with open pollination as compared to “closed” heads.  But only two of the 10 hybrids showed a significant increase in seed weight.  In South Dakota, the open-pollinated heads had a 27% increase in seed weight, with four hybrids receiving significant increases.  The differences were even greater in western Nebraska, where a 73% increase in seed weight occurred with open pollination, and eight of the 10 hybrids showed significant increases.  “All these results were from a model that controlled for flower head size,” Mallinger notes.  “So this takes into account [the reality] that some flowers are bigger than others.” The differences among the three states in seed mass on “closed” heads points to differing levels of self-fertility.  Comparing North Dakota with the other states, “in general, self-fertility seemed lower for some hybrids [in South Dakota and Nebraska], and this may be why we saw greater pollinator benefits there,” Mallinger says. 
  • The differences in the level of pollinator benefits among hybrids can be at least partially explained by bee visitation rates.  In North Dakota, “we saw significant differences across hybrids” in visitation rates,” Mallinger observes.  Not surprisingly, the higher the visitation rate, the greater the benefits of insect pollinators.  Visitation rates in South Dakota were quite low for all hybrids, whereas in Nebraska there was substantial variability among hybrids, which contributed to variability in pollinator benefits across the hybrids. “Even if we [account for] the number of flower heads in bloom, we see different visitation rates across states and hybrids,” Mallinger summarizes.  “For the hybrids, this could be due to variation in floral traits; some being more attractive to pollinators than others.”
  •  “Another way to look at pollinator benefits is to compare bee visitation rates to the yields from the unbagged (open-pollinated) heads,” the USDA entomologist states.  The 2016 findings, on average, showed a 19% increase in seed mass (weight) with increasing numbers of bee visits.  “So the more bee visits you have, the greater your yield.  No other studies have shown this direct relationship between the number of bee visits and yields in confection sunflower,” Mallinger adds.
  • The USDA?project also examined the types of bees visiting the sunflower sites in the three states.  Large-bodied solitary bees (including a few “sunflower specialists”) were by far the most common type.  Bumble bees were a distant second, followed by small-bodied sweat bees (often mistaken for wasps) and then metallic green sweat bees.  “Least common were the honey bees — this despite having honey bees ‘on property’ in North Dakota,” Mallinger says.  “South Dakota and Nebraska didn’t have them on site; but honey bees have a very wide foraging range from the hive — up to two miles or more.  So if we don’t see visits, it suggests they’re either not attracted to sunflower or are visiting other plants in the area.”
         Mallinger, Prasifka and their cooperators in South Dakota and Nebraska plan to repeat the 2016 study this season to gather another year’s data.   “We want to continue comparing pollinator benefits and degrees of self-fertility across hybrids and across environments,” Mallinger notes.  “We also plan to do experiments in growth chambers to test whether different temperatures affect plant self-fertility, which may explain why self-fertility varies across the three states.  We will also conduct hand self-pollination experiments to specifically examine degrees of genetic self-compatibility; that is, what does seedset look like when we apply self-pollen.”  The entomologists also hope to do additional evaluation of why some hybrids are more attractive to pollinators than others, and compare the pollinator communities across the three states.
— Don Lilleboe
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