Evaluating foliar insecticides and economic thresholds for Tychius picirostris (Coleoptera: Curculionidae) management in Oregon white clover seed production

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

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/17/2024
Publication Date: 7/13/2024
Citation: Tiwari, G., Kaur, N., Anderson, N.P., Lightle, D., Tanner, C.K., Donovan, B., Willette, A., Dorman, S.J. 2024. Evaluating foliar insecticides and economic thresholds for Tychius picirostris (Coleoptera: Curculionidae) management in Oregon white clover seed production. Journal of Economic Entomology. https://doi.org/10.1093/jee/toae163.
DOI: https://doi.org/10.1093/jee/toae163

Interpretive Summary: The clover seed weevil is a serious pest of white clover seed crops in the Willamette Valley, Oregon. Overreliance on pyrethroid insecticides for clover seed weevil control has increased insecticide resistance selection in clover seed weevil populations. Current management practices rely heavily on chemical control with foliar insecticides; however, limited modes of action (MoAs) are available to the industry to reduce pyrethroid insecticide use for effective insecticide resistance (IRM) strategies. This study evaluated the efficacy and timing of eight insecticide products targeting adult and larval clover seed weevil populations over the 2022 and 2023 growing seasons. Increased clover seed weevil abundance correlated with reduced seed. As expected, the pyrethroid MoA (bifenthrin, Brigade, IRAC group 3A) showed negligible adult and larval suppression due to insecticide resistance. Applications of anthranilic diamides (cyantraniliprole, Exirel, group 28; and isoxazolines, Plinazolin, group 30) reduced adult and larval populations when applied at pre-bloom and full-bloom growth stages. While suppression in clover seed weevil abundance from insecticide use was observed, the different insecticides did not produce different seed yields in 2022; Exirel product applied at full bloom growth stage in the 2023 trial was the only treatment combination that increased yield above the untreated control. Additional commercial fields were surveyed to compare larval detection methods. Results from standard Berlese funnels and a grower’s do-it-yourself (DIY) trap design provided similar estimates of larval densities. These findings demonstrate effective MoAs and application timing for clover seed weevil control and detection methods to establish an IRM program for clover seed weevil populations in white clover seed crops.

Technical Abstract: The clover seed weevil, Tychius picirostris Fabricius (Coleoptera: Curculionidae), is a significant pest of white clover seed crops in the Willamette Valley, Oregon. Overreliance on synthetic pyrethroid insecticides over the past decade has increased insecticide resistance selection pressure on T. picirostris populations in the region to pyrethroid active ingredients. Current management of T. picirostris in white clover grown for seed relies heavily on chemical control with limited modes of action (MoAs) available for effective insecticide resistance management (IRM) implementation. This study evaluated the efficacy and timing of eight foliar insecticide formulations (IRAC groups 1B, 3A, 22, 28, and 30) targeting adult and larval T. picirostris populations in replicated small and large plot field trials on commercial farms during the 2022 and 2023 growing seasons. Weekly adult and larval abundance in field plots were determined with sweep net sampling and Berlese funnel extractions, respectively. Large plot trials were harvested with a commercial combine at maturity, and seed yield was measured using a weigh wagon. The pyrethroid MoA (bifenthrin, Brigade, group 3A) showed negligible adult and larval suppression. Applications of anthranilic diamides (cyantraniliprole, Exirel, group 28; and isoxazolines, Plinazolin, group 30) reduced adult and larval populations when applied at pre-bloom (BBHC 59-60) and full-bloom growth stages (BBHC 65-66). Increased T. picirostris abundance correlated with reduced seed yield (R2 = 0.14, p<0.001) in large plot trials across two years. While suppression in T. picirostris abundance was observed among treatments, yield differences were insignificant in 2022; Exirel product (group 28) applied at BBHC 65-66 in the 2023 trial was the only treatment combination that increased yield above the untreated control. Additional commercial fields were surveyed to compare larval detection methods. Trap comparisons between standard Berlese funnels and a grower’s do-it-yourself (DIY) trap design were highly correlated and provided similar estimates of larval densities (R2 = 0.83, p<0.001). These findings demonstrate effective MoAs for T. picirostris control, optimal insecticide application timing, and detection methods for incorporation into a T. picirostris IRM program in white clover seed crops.