Field-evolved resistance of Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) to emamectin benzoate in Puerto Rico

The fall armyworm (FAW), Spodoptera frugiperda< (J.E. Smith), is an invasive species that causes significant damage to maize and has over 300 host plant species. There are several strategies to manage FAW, but synthetic insecticides are the primary tools in crop protection from FAW. It has developed resistance to more than 45 active ingredients, which poses a challenge to the sustainability of these synthetic management tools. Emamectin benzoate is a potent semi-synthetic insecticide widely used to manage FAW. In Puerto Rico, emamectin benzoate is one of the most effective compounds for managing FAW. However, its efficacy has been reduced in recent years. In this study, we determined the decreased susceptibility to emamectin benzoate in FAW. We performed overlay-diet assays in third-instar larvae of a FAW population from corn fields in Puerto Rico collected in 2023 and compared them with a susceptible colony. Mortality was scored after four days of treatment, and the data was analyzed by Probit Analysis (SAS). We found that the FAW from Puerto Rico has developed a remarkable over 22,000-fold resistance to emamectin benzoate that had resulted in “practical resistance”. To deal with this issue, the seed industry has implemented insecticide rotation and other integrated pest management (IPM) strategies. These findings are vital for Puerto Rico and the worldwide, emphasizing the importance of implementing preventive measures before resistance becomes widespread and establishing a sustainable approach to managing FAW globally.