Clover seed weevil is a serious insect pest that impacts Willamette Valley's white clover seed production. Historically, clover seed growers have relied heavily on broad-spectrum pyrethroid insecticides, including bifenthrin, as a management practice. However, since 2017, white clover seed growers have anecdotally reported poor efficacy and control failures with pyrethroid insecticides.
In response, the Oregon State University weevil research team — which includes Oregon State University Extension Service faculty, entomologists with the U.S. department of Agriculture’s Agricultural Research Service and graduate researchers — launched an intensive research and education program to assess resistance levels and identify effective alternatives.
Laboratory testing during the 2022–2023 seasons confirmed that clover seed weevils had developed high levels of resistance to bifenthrin, with resistant populations tolerating 100 to 700 times more bifenthrin than susceptible populations. Field trials also evaluated several alternative insecticides with reduced risk to pollinators and beneficial insects.
This work is helping growers avoid ineffective pesticide applications, reducing unnecessary environmental exposure and enabling a more sustainable, economically viable pest management program.
Steward, an insecticide with a novel mode of action, was tested and found effective in controlling the pest. Steward was recently approved 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.
The OSU Extension team emphasizes the importance of insecticide rotation and integrated pest management (IPM) strategies, particularly targeting the larval life stage for more effective long-term control.
In collaboration with the Oregon Clover Commission, the team presented these results at four grower meetings and one OSU Extension-led workshop. Three Extension seed production reports were made available during 2020-2023. These provide growers with yearly progress on the project.
Workshop evaluations revealed that none of the attendees planned to continue using bifenthrin as they had before. Two-thirds said they would no longer use it at all, and 78% adopted the monitoring methods demonstrated during the training. One field agronomist reported marked improvements in control during the 2023 season after implementing the new recommendations.
A peer-reviewed article was published in the Journal of Economic Entomology. The research 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
This work is helping growers avoid ineffective pesticide applications, reducing unnecessary environmental exposure and enabling a more sustainable, economically viable pest management program. The project is supported by Western SARE, the Oregon Clover Commission, and the OSU Agricultural Research Foundation.
