Susceptibilidad de Rachiplusia nu (Lepidóptera: Noctuidae) a la proteína Cry1A(c) expresada en soja Bt Y a insecticidas

Rachiplusia nu is one of the main soybean pests in Uruguay. Its control relies on the application of insecticides and the use of Bt soybean expressing the Cry1A(c) protein. Currently, Bt soybean accounts for approximately 20% of the cultivated area; however, resistance detected in R. nu populations in neighboring countries raises concerns about its effectiveness in Uruguay. Furthermore, failures in chemical control observed in different regions suggest variations in insecticide susceptibility. This study had two objectives: (1) to compare survival, foliar consumption, and feeding indices of R. nu larvae fed on Bt and non-Bt soybean, and (2) to assess larval mortality after exposure to leaves treated with chlorantraniliprole, triflumuron, or emamectin benzoate. The study was conducted at INIA La Estanzuela (Colonia) using larvae obtained from adults collected at the Dr. Mario A. Cassinoni Experimental Station (Paysandú). To evaluate feeding performance, third-instar larvae were individually placed in cages containing a leaflet from either Bt soybean (DM 5958 RSF IPRO) or non-Bt soybean (Génesis 5501). Insecticide susceptibility was assessed by immersing non-Bt soybean leaves in solutions containing water or insecticides at three concentrations (recommended dose, half dose, and double dose), following validated protocols (Insecticide Resistance Action Committee, IRAC). These leaves were then offered to second- and third-instar larvae, and mortality was recorded. Results from the Bt soybean assay showed that after four days of feeding, larval survival was 100% in both soybean types. Defoliation was lower in Bt soybean (9.91%) compared to non-Bt (17.4%); however, total foliar area consumed was similar in both treatments. No differences were observed in the growth rate, although larvae feeding on Bt soybean exhibited greater efficiency in food conversion. Relative consumption was higher in non-Bt soybean, while leaf digestibility did not differ between treatments. Since the soybean varieties used were not isogenic, these results may reflect confounding effects unrelated to the transgenic event. Regarding insecticide susceptibility, emamectin benzoate caused 100% mortality within 48 hours, while chlorantraniliprole reached this level by 94 hours. Triflumuron showed 70% efficacy at 48 hours and 90% at 96 hours. The findings suggest a loss of susceptibility of R. nu to Bt soybean and demonstrate effective control with the evaluated insecticides when applied at recommended doses. These results underscore the importance of monitoring both Bt and non-Bt crops and highlight the need to optimize insecticide application quality to ensure adequate pest exposure to the active ingredient.