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Red Seed Weevil in the Dakotas
Saturday, March 1, 2025
filed under: Insects
This troublesome insect pest of sunflower hasn’t gone anywhere. What does that imply for acreage & management options in 2025?
Photo credit: Patrick Beauzay / NDSU Entomology
USDA will release its first 2025 sunflower plantings forecast on March 31. That’s two-plus weeks after this article is being written.
For South Dakota, one of the biggest questions prior to the planting season is, to what degree will the past couple years of heavy red sunflower seed weevil (RSSW) pressure impact the 2025 planted acreage level? That question carries even more weight given that the RSSW has developed resistance to the pyrethroid class of insecticides in central South Dakota, the state’s main sunflower region.
The red seed weevil is not, of course, the only factor influencing the level of sunflower acreage in South Dakota this year. Current strong oil sunflower seed market prices — almost double that of this time a year ago — are obviously a major consideration in growers’ decisions. So too is the need for intervals between sunflower crops on the same ground.
Following a very tough RSSW year in 2023 for South Dakota, 2024 didn’t prove much easier. “The observed populations continue to be upwards of 500 weevils per plant,” notes Adam Varenhorst, extension entomologist for South Dakota State University. “The continued pressure from these large populations, coupled with the lack of effective insecticide options, has [had] a very negative impact for sunflower production in South Dakota.”
The acreage tally told the story. South Dakota farmers planted about 279,000 acres of sunflower in 2024 — compared to an average of 587,000 during the five-year period of 2017-2022. The 2023 acreage was 495,000. “There were likely more factors than just the red sunflower seed weevil causing this decline (e.g., market prices); but the red seed weevil isn’t helping,” Varenhorst affirms. “I would anticipate continued declines” until the RSSW situation improves, he adds.
(Due to this year’s attractive market, some industry insiders envision a significant increase in acreage in “non-traditional” sunflower areas of the state. But “traditional” areas — i.e., central South Dakota — are likely another story.)
The confirmation of pyrethroid-resistant RSSW in the state is a very big deal. Also, “recent studies suggest that in South Dakota, cross-resistance within the entire pyrethroid class is likely occurring,” Varenhorst says. “This indicates that pyrethroid insecticides should not be used for red sunflower seed weevil management in South Dakota.” Given the loss of chlorpyrifos (e.g., Lorsban and generics), that leaves virtually no chemical options.
“Early planting is our best option for red sunflower seed weevil management in South Dakota during 2025,” Varenhorst states. “This won’t completely remove weevil damage; but it will greatly reduce it.” There remains the possibility of emergency exemptions for the use of other insecticides, he adds. Should that come to pass, “the labels must be followed — and the highest labeled rate of the product should be used,” the SDSU entomologist emphasizes. “In addition, aerial applicators should not exceed the allowed applications of the product in an individual field during the growing season.”
Fortunately for North Dakota sunflower producers, the RSSW problem has not been nearly as intense as in their neighboring state. But it can’t be ignored, either — especially in areas of the state adjacent to South Dakota.
During 2024’s IPM Crop Survey in North Dakota, scouts evaluated 90 sunflower fields in 23 counties. The average number of seed weevils per head ranged from one to 25, with 71% of the weevils found along the field edge and 29% found within the field. During flowering, 47% of the surveyed fields had weevil infestations above the economic threshold.
Janet Knodel, professor and extension entomologist with North Dakota State University, offers these positives for her state’s growers: (1) As indicated by the 2024 IPM survey, RSSW populations in North Dakota have not reached anywhere near the levels further south, and (2) to date, there is no evidence of North Dakota seed weevil populations having resistance to the pyrethroid class of insecticides.
Knodel’s team conducted extensive bioassay testing for pyrethroid-resistant weevils in 26 fields in 11 counties — encompassing several districts — of North Dakota during 2019 to 2024. Results indicated that North Dakota’s RSSWs still remain susceptible to pyrethroid insecticides at all tested fields.
Still, Knodel encourages North Dakota sunflower producers — especially those in areas where populations have exceeded economic thresholds in recent years — to remain vigilant. That starts with timely, careful field scouting. Knodel suggests growers and crop consultants begin scouting for RSSW at the R4 (onset of petal emergence) stage of plant development. As noted in NDSU’s North Dakota Field Crop Insect Management Guide, insect counts should continue until the economic threshold level has been reached or most plants have attained 70% pollen shed.
“When sampling, use the X pattern and begin counting at least 70 to 100 feet into the field to avoid field margin effects,” the management guide states. “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, we recommend that treatment be considered when three out of 10 plants are just beginning to shed pollen.”
Though RSSW resistance to pyrethroids has not yet been confirmed in North Dakota, Knodel still recommends using the highest labeled rate of insecticide. Also, “if you have dry, droughty conditions, use an adjuvant to improve the chances of the insecticide droplets reaching the sunflower face,” she adds.
Both Knodel and SDSU’s Adam Varenhorst point to early planting as being one way to reduce RSSW problems. That idea was underscored by Jarrad Prasifka, USDA-ARS research entomologist, during the 2025 Sunflower Research Forum in January. He outlined research his group has been conducting on earlier planting in South Dakota.
Prasifka conducted research trials at Sturgis (western South Dakota) and Pierre (central South Dakota) in both 2023 and 2024, looking at planting dates and early vs. late-maturing hybrids. At Sturgis in 2023, the earlier planting dates (early, mid- and late May) all resulted in significantly lower red seed weevil populations compared to the later planting date (mid-June). Final seed yields likewise were significantly higher for the May planting dates. In 2024 at Sturgis, the early maturing hybrid incurred significantly less seed weevil damage across all four planting dates, compared to the late-maturing one.
The Pierre location experienced lower weevil populations and higher yields with the earlier planting dates in 2023; however, comparison of the early vs. late hybrids did not show much difference.
As to planting dates in 2024 at Pierre, the two early dates (April 25 and May 20) had much lower weevil damage than did the two late ones (June 6 and June 19). However, the highest seed yield at Pierre last year was with the early maturing hybrid planted on June 6. Prasifka summed the 2024 Pierre experience thusly: (1) the late April planting date was definitely too early, and (2) the mid-May planting date lowered weevil populations and preserved yield. Weevil emergence was late in 2024, however, so planting date’s overall effect was compromised.
So the bottom line on earlier planting is that while it’s no “silver bullet,” it does help avoid red sunflower weed weevil peak emergence and feeding injury. The downside for many growers, however, remains the logistics of planting sunflower during what is typically already a very busy time of year.
But in South Dakota, it may still be the best option until new labeled insecticides come along to manage the red sunflower seed weevil. — Don Lilleboe