Managing insecticide resistance in clover seed weevil through collaborative research

Tychius picirostris (clover seed weevil, CSW) is a key insect pest of white clover (Trifolium repens) in Oregon, causing substantial seed yield losses through larval feeding. For decades, CSW was primarily managed using bifenthrin (IRAC Group 3A), but toxicology bioassays conducted in 2022–2023 confirmed high levels of resistance (RR₅₀ = 178.00–725.67). Moderate resistance to malathion (Group 1B; RR₅₀ = 7.80–32.80) was also observed, prompting research into alternative modes of action (MoAs) and resistance management strategies. Large-plot field trials conducted from 2022 to 2025 across multiple commercial white clover seed fields evaluated insecticide efficacy and optimal application timing. Isocycloseram (Group 30) and cyantraniliprole (Group 28) consistently suppressed both adult and larval CSW populations. Sequential applications of malathion followed by chlorantraniliprole also provided effective adult control. Based on these findings, revised management recommendations include: (1) applying contact insecticides during spring adult migration when sweep net counts reach ≥2 weevils, and (2) applying systemic insecticides at full bloom when larval counts reach ≥3 per 30 inflorescences. Ongoing research is evaluating additional MoAs and pyrethroid-synergist combinations, guided by the toxicological evidence suggesting cytochrome P450 enzymes may contribute to CSW resistance.