Determining the efficacy of pyrethroid insecticides for red sunflower seed weevil (Coleoptera: Curculionidae) management in the Dakotas

In South Dakota, the red sunflower seed weevil is the most economically damaging insect pest of commercial sunflowers. Larvae of the red sunflower seed weevil feed directly on the seed kernel and reduce seed weight and oil content. Due to the impact of the red sunflower seed weevil on both oilseed and confection sunflower varieties, management of this pest is rigorous. It relies solely on the use of aerially applied broad-spectrum insecticides. In 2017, entomologists at South Dakota State University received field reports of pyrethroid insecticide failure in managing red sunflower seed weevils. The number of failure reports continued to increase during subsequent years. To determine the susceptibility of red sunflower seed weevils to pyrethroid insecticides, a glass scintillation vial assay was conducted. Red sunflower seed weevil populations from North Dakota and South Dakota were collected and evaluated for mortality to commonly applied pyrethroid active ingredients. In North Dakota, there were no differences in mortality among the insecticide treatments. In South Dakota, resistance to lambda-cyhalothrin, esfenvalerate, and cypermethrin was observed. In 2022, the addition of piperonyl butoxide enhanced the efficacy of esfenvalerate, suggesting that the mechanism responsible for resistance involves the cytochrome P450 system.