Evaluating new insecticides to manage clover seed weevils in white clover seed crops – 2022

Authors: TIWARI, GRACE - Oregon State University; KAUR, NAVNEET - Oregon State University; LIGHTLE, DANI - Oregon State University; ANDERSON, NICOLE - Oregon State University; TANNER, CHRISTY - Oregon State University; Dorman, Seth

Submitted to: Seed Production Research at Oregon State University
Publication Type: Other
Publication Acceptance Date: 7/3/2023
Publication Date: 7/3/2023
Citation: Tiwari, G., Kaur, N., Lightle, D.M., Anderson, N.P., Tanner, C.K., Dorman, S.J. 2023. Evaluating new insecticides to manage clover seed weevils in white clover seed crops – 2022. Seed Production Research at Oregon State University. Pgs. 3-6. https://cropandsoil.oregonstate.edu/sites/agscid7/files/crop-soil/2022_seed_production_combined_final_report.pdf

Interpretive Summary: The clover seed weevil (CSW) is a serious pest in white clover seed production in Oregon’s Willamette Valley that can significantly reduce seed yield and economic returns. Current recommendations for the timing of foliar spray applications to control CSW are based on adult activity and do not consider the severity of crop-damaging larval infestations. In the present study, we hypothesized that using more diverse modes of action applied at first larval detection targeting CSW larvae will reduce feeding damage and seed yield loss. Our primary objective was to evaluate the efficacy of newer insecticide chemistries for CSW management in commercial white clover seed fields when applied at first larval detection compared to a standard practice of applying pyrethroids (Brigade) at BBCH 65-66. One large-scale field trial was conducted in a commercial white clover seed field in Linn County, Oregon, in 2022. Nine treatments, including an untreated control, were evaluated for CSW control and seed yield. Adult CSW abundance using sweep net sampling differed among insecticide treatments, and there was an interaction between treatment and sampling date factors. Across all sampling dates, CSW adult counts in plots treated with Steward, Malathion + Brigade, and Harvanta + Brigade formulated products were the only treatments lower than Brigade alone. The number of larvae per 30 inflorescences was not different among treatments, and no differences were detected among treatments at each sampling date. Brigade did not provide adequate control of adult or larval populations. In summary, limited efficacy of Brigade to reduce CSW adult and larval populations in a field trial aligns with preliminary laboratory assays suggesting high levels of pyrethroid resistance with local CSW populations using technical-grade chemistry and formulated product. No differences in total clean seed yield were observed among treatments in 2022. Similar field trials will be conducted in 2023 to further evaluate insecticide treatments on seed yield and determine recommendations regarding insecticide chemistry selection for CSW control in Oregon white clover seed production.

Technical Abstract: The clover seed weevil (CSW), Tychius picirostris Fabricius (Coleoptera: Curculionidae), is one of the primary factors limiting seed yield potential in white clover (Trifolium repens L.) crops in the Willamette Valley, Oregon. High selection pressure of broad-spectrum insecticides has significantly increased the risk of insecticide resistance development in CSW populations. Current action thresholds for foliar insecticide applications are based on adult activity and do not consider the severity of larval infestations. In the present study, we hypothesized that using more diverse modes of action applied at first larval detection targeting CSW larvae will reduce feeding damage and seed yield loss. Our primary objective was to evaluate the efficacy of newer insecticide chemistries for CSW management in commercial white clover seed fields when applied at first larval detection compared to a standard practice of applying pyrethroids (Brigade) at BBCH 65-66. One large-scale field trial was conducted in a commercial white clover seed field in Linn County, Oregon, in 2022. Nine treatments, including an untreated control, were evaluated for CSW control and seed yield. Adult CSW abundance using sweep net sampling differed among insecticide treatments, and there was an interaction between treatment and sampling date factors. Across all sampling dates, CSW adult counts in plots treated with Steward, Malathion + Brigade, and Harvanta + Brigade formulated product were the only treatments lower than Brigade alone. The number of larvae per 30 inflorescences was not different among treatments, and no differences were detected among treatments at each sampling date. Brigade did not provide adequate control of adult or larval populations. In summary, limited efficacy of Brigade to reduce CSW adult and larval populations in a field trial aligns with preliminary laboratory assays suggesting high levels of pyrethroid resistance with local CSW populations using technical-grade chemistry and formulated product. No differences in total clean seed yield were observed among treatments in 2022. Similar field trials will be conducted in 2023 to further evaluate insecticide treatments on seed yield and determine recommendations regarding insecticide chemistry selection for CSW control in Oregon white clover seed production.