Management of pea leaf weevil (Coleoptera: Curculionidae) and development of an economic threshold in faba beans

Pea leaf weevil (PLW), (Sitona lineatus Linnaeus; Coleoptera: Curculionidae), is an invasive pest in North America. Pea leaf weevil causes economic damage to field pea (Pisum sativum) and faba bean (Vicia faba). Adults feed on the foliage and larvae feed on root nodule-associated Rhizobium bacteria. We developed an economic threshold for PLW in faba bean and tested the efficacy of seed-coated (thiamethoxam) and foliar (λ-cyhalothrin) insecticides in managing pea leaf weevil. A field plot study was conducted at two sites in Alberta Canada, in three study years. The treatments 1) fungicide seed treatment, 2) fungicide + thiamethoxam, 3) fungicide + λ-cyhalothrin at 2^nd node stage, 4) fungicide + λ-cyhalothrin at 2^nd and 4^th node stages and, 5) fungicide + thiamethoxam + λ-cyhalothrin at 2^nd node stage were assigned to plots, following a randomized block design. Plants were assessed for adult and larval damage and yield. Pea leaf weevil feeding reduced faba bean yield and thiamethoxam protected nearly 1 tonne/ha of yield. Foliar insecticide was not effective in protecting yield. The economic injury level for PLW was 15% of seedlings with terminal leaf damage. Thiamethoxam reduced both adult and larval damage. In conclusion, terminal leaf damage could be used to predict faba bean yield loss caused by the pea leaf weevil. The seed-coated insecticide currently registered in Canada protects faba bean yield, but there is a need for more Integrated Pest Management research to develop strategies to reduce insecticide dependency.