Susceptibility of Spodoptera cosmioides to insecticides with different modes of action: implications for integrated pest management

Abstract

The use of insecticides is essential for managing soybean pests, particularly under high infestation pressure. Spodoptera cosmioides (Walker) (Lepidoptera: Noctuidae), a polyphagous lepidopteran species widely distributed in South America, has increased in abundance in recent years, raising concerns about its economic impact and insecticide susceptibility. This study evaluated the efficacy of commercial formulations containing single or dual active ingredients, applied through ingestion or contact to simulate field exposure. Additionally, susceptibility to fipronil was compared among S. cosmioides, Rachiplusia nu (Guenée), and Chrysodeixis includens (Walker) using contact bioassays. These insecticides are widely used in Argentina to control soybean defoliators and weevils. In ingestion bioassays, chlorpyrifos was approximately 50-fold more toxic than thiamethoxam + lambda-cyhalothrin at 48 h post-treatment. At 72 h, spinosad showed the lowest efficacy. At 96 h, chlorantraniliprole alone and combined with lambda-cyhalothrin were 25.5- and 17-fold more toxic, respectively, than chlorantraniliprole + abamectin. In contact bioassays, acetamiprid + esfenvalerate and thiamethoxam + lambda-cyhalothrin showed similar toxicity, whereas fipronil exhibited species-specific efficacy. Rachiplusia nu was less susceptible to fipronil than the other species. These findings emphasize the need for continuous susceptibility monitoring in S. cosmioides to support resistance management and sustain integrated pest management programs in soybean crops.