LAUSR

Simple Summary The fall armyworm (FAW), Spodoptera frugiperda (J.E Smith), is an important insect pest that can cause severe damage to a wide range of economically important crops. Many insecticides are used to manage the FAW in Brazil, including teflubenzuron. Here, we monitored the susceptibility of the FAW to teflubenzuron in more than 200 field-collected populations from major corn-growing regions of Brazil, from 2004 to 2020. Overall, our findings showed that the FAWs susceptibility to this insecticide reduced over the years, providing evidence of field-evolved resistance. We discuss the possible causes for this loss in the susceptibility to teflubenzuron in S. frugiperda and focus on the importance of implementing resistance management strategies. Abstract Susceptibility monitoring to insecticides is a key component to implementing insecticide resistance management (IRM) programs. In this research, the susceptibility to teflubenzuron in Spodoptera frugiperda (J.E Smith) was monitored in more than 200 field-collected populations from major corn-growing regions of Brazil, from 2004 to 2020. Initially, we defined a diagnostic concentration of 10 µg mL−1 of teflubenzuron using a diet-overlay bioassay for monitoring the susceptibility. A variation in the susceptibility to teflubenzuron in S. frugiperda was detected among populations from different locations. We also detected a significant reduction in the susceptibility to teflubenzuron throughout time in all the populations of S. frugiperda evaluated, with larval survival at diagnostic concentration varying from values of <5% in 2004 to up 80% in 2020. Thus, this research provides evidence of field-evolved resistance of S. frugiperda to teflubenzuron and reinforces that IRM practices are urgently needed to be implemented in Brazil.