Assessing chemical control options and their effects on the lesser clover leaf weevil (Hypera nigrirostris) and red clover pollinators in Western Canada

Single-cut red clover (Trifolium pratense L.) is an important forage seed crop in Western Canada due to its winter hardiness and economical value. However, infestation of lesser clover leaf weevil (Hypera nigrirostris F.) can lead to drastic yield losses (over 50%). The aim of this research was to evaluate the effectiveness of insecticides deltamethrin (Decis) and cyantraniliprole (Exirel) in controlling H. nigrirostris in red clover in both field and laboratory conditions and their impacts on the pollinator community. Pest pressure data in red clover fields was tracked during the 2018 and 2019 growing season in six locations. Under the field conditions both deltamethrin and cyantraniliprole were effective at controlling H. nigrirostris numbers. However, only deltamethrin provided rapid pest suppression in 24 h after treatment application. Rearing H. nigrirostris larvae on treated plants in laboratory conditions showed similar results where both insecticides significantly reduced H. nigrirostris numbers in 10-12 days after treatment. The abundances of H. nigrirostris was always negatively related to seed yield regardless of treatment and initial weevil pressure. Seed yield was significantly lower in untreated controls whereas no differences were found between deltamethrin and cyantraniliprole. In contrast, no yield response was found when H. nigrirostris abundance was lower than 4 larvae per 10 shoots. Unexpectedly, densities of bees were not affected by either treatment and were not related to seed yield. Overall, results indicate that both insecticides can provide sufficient H. nigrirostris control, and that pollinator densities are not limiting seed yield, whereas lack of H. nigrirostris management can cause substantial yield losses.