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Will 2023 Bring a Red Seed Weevil Rematch?

Thursday, March 30, 2023
filed under: Insects

Year’s Experience, Especially in Central & Western South Dakota, Calls for Close Scouting & Timely Treatment Should Economic Thresholds Be Approached or Topped
seed weevils
Photo: Patrick Beauzay / NDSU Entomology
No one needs to tell sunflower producers in central and western South Dakota about the costly red sunflower seed weevil (RSSW) problem.  It’s been a troublesome insect in the area for a number of years — with 2022 ranking at or near the top. 
        “In recent years, the red sunflower seed weevil populations have been higher than normal in many areas of South Dakota,” wrote South Dakota State University entomologist Adam Varenhorst last August.  “In some areas, the populations have far exceeded the economic threshold (four to six adults per head on oil-type ’flowers) with 300 all the way up to an estimated 5,000 adults per sunflower head.  The 2022 populations that have been observed in several fields were well over the economic threshold.”
        Chuck Todd was among those impacted.  “Sunflower weevil damage [in 2022] was high in my area and high in my own fields,” says the Sully County producer.  “We planted on June 1 with two different maturities of plants; sprayed late for one variety and early for the second variety.”  Weevil numbers ranged between 10-15 per head at the first spraying; by the time of the second application seven days later, populations ran between 30 to 50 seed weevils per head.  The Onida grower estimates that “about 30% of the seeds were damaged to some degree with weevils.”
        As costly as a 30% seed damage level is, the red seed weevil pain can go much deeper, as Varenhorst noted. Under severe infestations, previous studies documented 50-80% of achenes (seeds) being damaged.  However, in a 2022 study Varenhorst observed 96% achene damage.  Seed weevil larvae commonly consume about one-third of the kernel, impacting both yield and test weight.  The problem is even more sensitive in confection sunflower, given its human consumption destination.  There, the economic threshold is just one seed weevil per head, compared to the four to six threshold for oil-type ‘flowers.
     What has contributed to the high seed weevil populations of recent years in central South Dakota?  Several factors likely are at play:
     •  Recent summers that have been very warm and dry.
     •  Warmer winters, thus aiding overwintering weevil survival.
     •  Limited tillage, as most central South Dakota sunflower fields are no-till.
     •. More sunflower acreage in Sully and nearby counties.
     •  Increasing resistance to pyrethroid insecticides (more on this later).
     •  Late planting dates (more on this later as well).
      While there also have been several “hot spots” in North Dakota during the past several years, the red sunflower seed weevil has not been, state-wide, as economically damaging as in South Dakota.  There were only a handful of sampled sites in North Dakota in 2022 where the average number of weevils per head exceeded four.
seed weevil life cycle

        The main foliar insecticides used for control of the red sunflower seed weevil have been primarily the Group 3 pyrethroids (e.g.,  Asana,  Asana XL, Baythroid XL, Delta Gold, Lambda-Cy, Mustang Maxx, Tombstone and Warrior II); and minimal use of the premix Besiege (Coragen 28 + Warrior 3A).
        “In South Dakota, there are populations of red sunflower seed weevil that are resistant to the lambda-cyhalothrin (Warrior II), esfenvalerate (Asana XL) and zeta-cypermethrin (Mustang Maxx) active ingredients,” notes SDSU’s Varenhorst.  He advises South Dakota producers to not rely on these products for seed weevil management in 2023.  “Research at SDSU did observe an approximate 10% increase in the effectiveness of esfenvalerate when combined with a piperonyl butoxide synergist (Exponent, PBO-8) in a laboratory setting; but this may still result in above-threshold weevil populations, depending on how large the treated population was,” Varenhorst says.
        Field trials were conducted last year in both South Dakota and North Dakota to test the efficiency of several commonly used insecticides for RSSW control.  While the final South Dakota results were not available as of this writing, those from North Dakota (an oil-type hybrid, seeded at Casselton on May 24 at a population of 17,500/ac) indicated the following conclusions:
        •  There were no differences among all treatments for head diameter, indicating that heads were relatively uniform in size throughout the trial.
        •  All pyrethroids, pyrethroid + piperonyl butoxide combinations, and Entrust SC had significantly less RSSW seed damage compared to the untreated check and the other insecticide treatments.  Neemix, Vantacor, Dimate, Malathion and Sevin XLR Plus were not significantly different from the untreated check for RSSW seed damage.
        •  There were significant differences among treatments for test weight.  As could be expected, treatments with greater RSSW damage had lower test weight.
        •  Although all treatments outyielded the untreated check, none did so by a significant amount.
        Entomologists at both NDSU and SDSU will continue investigations in 2023 into RSSW resistance and efficacy of various labeled and unlabeled insecticides, as well as looking at other experimental products.  Evaluation of RSSW populations in South Dakota for cross resistance within the pyrethroid class of insecticides will be a priority.  NDSU’s Janet Knodel says her group also will work to get a better handle on the flight potential of the red sunflower seed weevil “to allow researchers to develop maps using previously collected insecticide bioassay data to determine areas that are of the greatest risk for insecticide failures.”  Finally, Knodel says, “the evaluation of insecticide products in the field efficacy studies will provide data that will be used to produce management recommendations — and then Section 18 registrations can he requested for unlabeled insecticides.”
        Consistently using the same active ingredient is a recipe for resistance development, the university entomologists state.  “Since we rely on chemical control as a key strategy for control of red sunflower seed weevils, we really need new insecticides and modes of action as soon as possible,” Knodel emphasizes. 
        In the meantime, Dakota sunflower producers needing to manage the red sunflower seed weevil in 2023 are strongly encouraged to consult the field crop insect management guide for their state.  For South Dakotans, go online with a Google search to “South Dakota 2023 Pest Management Guide - Alfalfa and Oilseeds.”  For the North Dakota publication, type in “2023 North Dakota Field Crop Insect Management Guide.”
        Another excellent information source during the growing season is NDSU Extension’s weekly “Crop & Pest Report.”  It can be found at production/crop-pest-report.  Simply enter your email address at the bottom of the CRP webpage for a free subscription.
        Knodel and Varenhorst encourage growers with past, consistent seed weevil problems to use alternative strategies like cultural control, and to move their planting date forward, if possible.  “Planting date has been shown to mitigate red sunflower seed weevil populations,” Knodel states.  For North Dakota growers with past significant seed weevil problems, she encourages an early to mid-May planting.  And, she adds, if South Dakota growers can move away from mid- or late June planting dates, that should contribute to better seed weevil control in that state. 
        Her observation is underscored by a 2022 trial at the SDSU Dakota Lakes Research Farm near Pierre.  Sunflower plots were planted on May 16, May 27, June 3, and June 17. The sunflower variety differed between the May 16 planting date and the other dates. The sunflower plots were not sprayed with any insecticides, and very large weevil populations (i.e., more than 500 red seed weevils per head) were observed infesting sunflower from all of the planting dates.
        At season’s end, sunflower heads from the Dakota Lakes plots were sampled and sent to the USDA-ARS Sunflower and Plant Biology Research Unit in Fargo, where they were analyzed. The seeds were x-rayed, and red sunflower seed weevil damage was noted for any seeds that had removed tissue (dark areas on the otherwise light seed).  Initial results indicated that earlier planting dates definitely resulted in reduced RSSW damage: the May 16 planting date had just 1% damage, the May 27 planting date had 23% damage, the June 3 planting date had 51% damage, and the June 17 planting date incurred 96% damage.
        Onida grower Chuck Todd is taking that to heart.  “This year, we plan on planting before June 1, depending on weather conditions,” he explains.  “We will spray at the 1% flowering time, wait four days and respray if necessary.” 
          The result, Todd anticipates, will be more yield and, ultimately, a bigger net profit at season’s end.  And in Sully County, there’s a lot of potential for big results.  Even with 30% of his 2022 crop incurring some red seed weevil damage, Todd’s sunflower still averaged 2,600 lbs/ac with a test weight of 31.5 lbs/bu. — Don Lilleboe

Scouting & Optimal Treatment Timing for Insecticide Control of Red Sunflower Seed Weevils
Source: NDSU Extension
        The red sunflower seed weevil (RSSW) begins to emerge in early July and continues until mid-August.  Peak emergence occurs in late July.  Start counting adult seed weevils when the yellow ray petals are just beginning to show.  Counts should continue until the economic threshold level has been reached or most plants have reached 70% pollen shed.  Fields where most plants are at that 70% pollen shed stage should no longer be susceptible to further significant damage, possibly due to the hardening of the outer seed shell preventing egg-laying by female weevils.
        When sampling for the red sunflower seed weevil, use the X pattern and begin counting at least 70 to 100 feet into the field to avoid field margin effects.  Count the number of weevils on five plants at each site, for a total of 25 plants.  The ideal plant stage for treatment is when most individual plants are at 40% pollen shed.  However, NDSU recommends that treatment be when more than 50% of the field’s plants are just beginning to show yellow ray petals (the R5.0 growth stage) to 30% of the heads shedding pollen (R5.3 stage) and the rest 20%  of plants are still in the bud stage.
        “This difference between the ideal plant stage (R5.4) to treat and the earlier plant stage (just beginning pollen shed, R5.1) is based in part on the fact that aerial applicators have a busy schedule, and may not be able to spray on time due to adverse weather,” NDSU entomologists note.  “Treating at the early bloom stage should allow growers a sufficient buffer of time to have their fields treated at the proper time.”
          But there’s also this caveat, NDSU entomologists point out:  “Growers must be aware, however, that if weevil populations are higher and/or spraying is done too early, a re-infestation may occur and a second insecticide application may be necessary.”
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