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The RSSW Battle

Saturday, March 23, 2024
filed under: Insects

A Review of South Dakota’s 2023 Struggles With the Red Sunflower Seed Weevil
— Significant Acreage Drop Likely in 2024 —

red seed weevils — Photo credit: Patrick Beauzay / NDSU Entomology

       It’s been a tough, tough couple of years when it comes to red sunflower seed weevil (RSSW) problems in central South Dakota — one of the most consistent sunflower production areas in the United States. Extremely high populations of this insect have led to dramatic reductions in both yield and test weight in numerous fields, resulting in big economic losses for producers. In some 2023 fields, in fact, the crop’s quality wasn’t even suitable to market.
        It’s not that sunflower growers have been lax in scouting and treating for this insect; just the opposite. Rather, a major reason for the exceedingly high RSSW populations and resulting damage has been the weevil’s developed resistance to the pyrethroid class of insecticides — e.g., Asana XL, Warrior II, Baythroid XL and Delta Gold.
        The trail of documented resistance in South Dakota actually goes back to 2017-18, when failures were first observed in some fields that had been treated with a pyrethroid. South Dakota State University entomologists observed reduced susceptibility in 2020 and 2021, with actual resistance confirmed in 2021, 2022 and 2023. Extreme resistance to Delta Gold was recorded in South Dakota’s Potter and Sully counties last year; resistance levels were less dramatic with Warrior II (though still way too high), followed at lower levels by Asana XL and Baythroid XL. The pattern was similar — though at lesser levels — at the Dakota Lakes Research Farm near Pierre. Red sunflower seed weevil resistance was not observed last year in SDSU trials at the West River Research Farm near Sturgis.
        Another big factor behind the high levels of RSSW damage in recent years was the EPA’s revocation in August 2021 of all food tolerances for chlorpyrifos (e.g., Lorsban and generics). The EPA action was in response to an April 2021 order from the U.S. Court of Appeals for the Ninth Circuit. Lorsban had been an effective “go to” product for a number of years, and its loss was keenly felt by affected producers of several crops, including sunflower.
        Recently, however, the U.S. Court of Appeals for the Eighth Circuit issued a ruling vacating the EPA’s final rule. This latest ruling means that currently registered insecticide products with the active ingredient chlorpyrifos can now be used on labeled crops during the 2024 growing season. (See more on this topic in the article on page 17.)
        Prior to the loss of chlorpyrifos in 2021, the threat to South Dakota sunflower from the RSSW “was effectively managed with chlorpyrifos tank-mixed with the pyrethroid insecticides,” points out Ryon Berry, field sales manager with Nuseed. He anticipates that the return of chlorpyrifos will help at least some central South Dakota growers “to produce a high-quality crop once again.”
        While South Dakota has been struggling with such excessive populations of RSSW and the resulting seed damage, North Dakota sunflower fields have fared significantly better. A 2023 survey by North Dakota State University entomologists of 169 sunflower fields in 35 counties showed average seed weevil populations of between one to 12 per head. Only 16% of the surveyed fields ranked above the economic threshold for RSSW. NDSU entomologists caution, though, that the state’s growers — particularly in counties adjacent to South Dakota — need to stay vigilant.

Will Early Planting Help?

seed samples chart — Seed Samples: 2023, by Insect Type
Credit: Jarrad Prasifka / USDA-ARS

       Utilizing an earlier sunflower planting date to avoid red seed weevil damage — at least partially — was shown to be effective during research in the 1980s. One reason the approach hasn’t been used too widely is simple logistics: Growers are already stressed for time and equipment earlier in the spring while planting corn, spring wheat and soybeans. Adding sunflower to that time window may not be feasible for many. Also, there are concerns about weed control not being adequate in early planted sunflower, i.e., not getting a good burndown on some of the later-emerging broadleaf weeds.
        In 2023, USDA-ARS research entomologist Jarrad Prasifka carried out a planting date study at three locations: Carrington, N.D., and Pierre and Sturgis, S.D. No insecticides were used in the study, which was supported by the National Sunflower Association. At Carrington, the four planting dates ranged from May 16 to June 13; at Pierre, from May 1 to June 15; and at Sturgis, from May 8 to June 20.
        In the planting date study — which will be continued in 2024 — Prasifka found that seed weevil damage at the Pierre site was about 70% lower in the early planting versus the later plantings; also, yield and oil content were equal or better in the early planting. Seed weevil emergence at this location occurred during the approximate one-month period of July 7 to August 7.
        At Carrington, there were no significant differences in RSSW damage between early versus late planting or between the two hybrids being compared (one an ultra-early Clearfield® type and the other a medium-maturity Clearfield). The medium-maturity hybrid yielded significantly better in all four planting dates at Carrington, but the differences were not attributable to seed weevil effect.
        Pierre area grower Mark Small, frustrated with the cost and poor results from insecticides, decided to try something new on his farm last year: earlier planting. Small had already cut his sunflower acreage in 2023 by about 1,000 acres from 2022 due to the red seed weevil and crop rotation considerations.
        Small’s overall 2023 experience with the RSSW was, in his description, “disastrous” — except for his earliest planted acreage. That field, planted on April 30, was harvested by the end of August, allowing him to plant back to winter wheat. It produced a strong test weight of 32 lbs/bu. His later-planted sunflower acreage actually had some pretty good yields, but test weights were poor overall, leading to the rejection of a majority of those loads at market — seeds that he was still holding as of mid-March.
        Small had planned to experiment again in 2024 with a limited acreage of early planted sunflower, but has since decided to omit the crop altogether this season from his rotation. Milo and corn will cover most of the ground that otherwise would have been seeded to sunflower.

        SDSU entomologist Adam Varenhorst sums up his current recommendations for red sunflower seed weevil this way:
        “Research from 2023 suggests that there is an overall breakdown of the pyrethroid class of insecticides in South Dakota. This means rotating active ingredients within that class will still not provide adequate population management for most red sunflower seed weevil populations.
        “Advice regarding the use of insecticides remains the same: (1) Apply the insecticides per the label recommendations and use the highest labeled rate. (2) Do not reapply the same active ingredient if it did not effectively reduce the pest population. (3) Do not exceed the label recommendations for the number of applications or AI/acre within a single season.”
        Varenhorst emphasizes the importance of the emergency exemption for South Dakota in 2024 for chlorpyrifos-containing insecticides. “However,” he concludes, “the greatest tool we have, based on research, is the use of an earlier planting date — a cultural control strategy — to reduce red sunflower seed weevil damage.”
        Adds Nuseed’s Ryon Berry: “Even with the return of the availability of chlorpyrifos insecticide in 2024, there are concerns that the number of insects that will be present could be so high that reinfestation after spraying an effective insecticide may still be an uphill battle to maintain grain quality at harvest time.” Still, the return
of chlorpyrifos — assuming product supply is adequate to meet demand — should go a long way toward growers in central South Dakota being able to counter the RSSW and produce a quality, profitable sunflower crop, Berry says. — Don Lilleboe