Susceptibility of Brazilian populations of Rachiplusia nu (Lepidoptera: Noctuidae) to insecticides and determination of diagnostic concentrations for a proactive resistance management program.

Abstract

The sunflower looper, Rachiplusia nu (Guenée), evolved resistance to the insecticidal protein Cry1Ac expressed in soybean and emerged as an important soybean pest in Brazil, requiring the application of insecticides for their control. Here, we characterized the susceptibility of Brazilian populations of R. nu to several insecticides and developed diagnostic concentrations for a proactive insect resistance management (IRM) program. Geographically distinct populations of R. nu from Brazil showed a small amount of variation in susceptibility to 15 growth regulator-, muscle-, nerve- and respiration-action insecticides, presenting resistance levels ≤6.2-fold those of a susceptible population. In contrast, field populations of R. nu showed 11- to 31-fold higher resistance to flubendiamide (muscle-acting) and bifenthrin (nerve-acting) than did the susceptible population, suggesting the presence of low to moderate resistance levels. The diagnostic concentration of each insecticide established in the current study caused >98% mortality against field populations of R. nu. In general, field populations of R. nu from Brazil showed a small amount of variation in susceptibility and low levels of resistance to major insecticides commonly used for their control. The estimated diagnostic concentrations defined here are recommended for use in a proactive IRM program for monitoring the resistance of R. nu to insecticides. © 2024 Society of Chemical Industry.