Oregon produces more than 90% of the nation’s white clover seed, supporting forage systems, soil health and agricultural productivity. Clover seed weevil is a serious insect pest that impacts the Willamette Valley’s white clover seed production.
By helping growers respond to insecticide resistance with science-based timing, monitoring and rotation strategies, Oregon State and USDA research reduces ineffective pesticide use, protects beneficial insects and extends the life of available pest management tools.
Historically, growers relied heavily on broad-spectrum pyrethroid insecticides, including bifenthrin, as a primary management tool. However, since 2017, growers have reported poor control and declining effectiveness.
In response, the Oregon State University weevil research team — including Oregon State University Extension Service faculty, U.S. Department of Agriculture Agricultural Research Service entomologists and graduate researchers — launched an intensive research and education effort to measure resistance and identify new strategies.
Laboratory testing during the 2022–2023 seasons confirmed that clover seed weevil populations had developed very high resistance to bifenthrin, tolerating up to 700 times higher exposure than susceptible populations.
Field trials from 2022 to 2024 in commercial white clover seed fields in Western Oregon refined how and when growers should manage the pest.
Researchers found that timing matters. Early-season applications, made before bloom, are most effective for controlling adult weevils. Mid-season applications, during full bloom, better target larvae feeding inside developing seeds — the stage responsible for the greatest yield and quality losses.
Across sites and years, newer insecticides showed differing strengths. Isocycloseram consistently reduced adult populations, while cyantraniliprole helped suppress larvae. Indoxacarb, previously identified as an effective option, showed more variable performance under field conditions.
Although seed yields did not differ significantly among treatments in these newer trials, the results were critical for identifying effective alternatives and supporting the registration of additional insecticide options for use in Oregon.
Rotation replaces single-product reliance
The research marks a shift away from reliance on a single product toward a more resilient, integrated approach. Based on these results, the researchers emphasize the importance of insecticide rotation and integrated pest management (IPM) strategies, particularly targeting the larval life stage for more effective long-term control.
Steward, an insecticide with a novel mode of action, was tested and found effective in controlling the pest. Steward was approved in 2025 under an U.S. Environmental Protection Agency Section 24(c) special local needs registration to enable its use in white clover seed production.
Seed yield trials showed Steward led to gains of $290 to $384 per hectare — a significant economic benefit. These returns were compared to untreated controls and highlight the value of switching to more effective chemistries.
Together, the findings show that improved timing and product selection can protect both yield and the long-term effectiveness of available tools.
Growers adopt new practices
In collaboration with the Oregon Clover Commission, the research team shared findings through four grower meetings and an OSU Extension workshop. Extension seed production reports published from 2020 to 2023 provided ongoing updates.
Workshop evaluations showed rapid adoption. None of the attendees planned to continue using bifenthrin as before, two-thirds said they would stop using it entirely and 78% adopted recommended monitoring practices. One field agronomist reported marked improvements in control after implementing the new approach.
A peer-reviewed article was published in the Journal of Economic Entomology, and the team published Management Guidelines for the Clover Seed Weevil in Oregon White Clover Seed Production.
The original OSU weevil research team included:
- Navneet Kaur, associate professor and Extension field crops entomologist
- Christy Tanner, associate professor of practice and Extension seed production agronomist
- Seth Dorman, USDA-ARS entomologist and OSU courtesy entomology faculty member
- Grace Tiwari, OSU graduate student
- Nicole Anderson, former Extension statewide seed production specialist
- Dani Lightle, former assistant professor of practice and OSU IR-4 specialty crops pesticide registration research leader
Public value
By helping growers respond to insecticide resistance with science-based timing, monitoring and rotation strategies, Oregon State and USDA research reduces ineffective pesticide use, protects beneficial insects and extends the life of available pest management tools.
This work strengthens the economic sustainability of Oregon’s forage seed industry and improves the return on public investment in agricultural research.
Continued investment in applied research ensures growers have practical, science-based solutions to emerging challenges like insecticide resistance. The work strengthens the long-term economic sustainability of Oregon’s white clover seed industry.
The project is supported by Western Sustainable Agriculture Research and Education — a program of the U.S. Department of Agriculture National Institute of Food and Agriculture — the Oregon Clover Commission, and the OSU Agricultural Research Foundation.