Variation in Label-Rate Insecticide Efficacy and Control Failure Risk Among Brazilian Populations of Spodoptera...

Spodoptera frugiperda (Lepidoptera: Noctuidae) is a key insect pest of maize in Brazil. However, the evolution of insecticide resistance has raised concerns about the risk of control failures under field conditions. This study assessed insecticide efficacy and estimated the risk of control failure in Brazilian populations of S. frugiperda exposed to selected insecticides. Nine field populations were collected from representative maize-producing regions of Brazil during the 2024/2025 and 2025/2026 seasons. Early third-instar (L3) larvae from these populations were exposed to the highest label rate of ten insecticides representing different modes of action in maize leaf bioassays. Mortality was assessed 96 h after exposure, and the risk of control failure was based on the efficacy threshold of 80% mortality. Significant effects of population, insecticide, and the population × insecticide interaction were detected, indicating variation in susceptibility among the sampled populations. Methoxyfenozide + spinetoram, spinosad, chlorfenapyr, and spinetoram consistently caused high mortality (>81%) and were associated with low risk of control failure across all populations. In contrast, chlorantraniliprole exhibited reduced efficacy with mortality below the expected efficacy threshold in most populations and a high risk of control failure. Population-dependent variation in insecticide efficacy was also evident for metaflumizone, emamectin benzoate, thiodicarb, indoxacarb, and methoxyfenozide, with corresponding variation in the risk of control failure. These findings show that insecticide efficacy and the risk of control failure depend on the specific insecticide-population combination and provide valuable information to support decision-making regarding insecticide selection and the rotation of modes of action within insect resistance management (IRM) and integrated pest management (IPM) programs.