Evidence of field-evolved resistance in Spodoptera frugiperda (Lepidoptera: Noctuidae) to emamectin benzoate in Brazil

The fall armyworm (FAW), Spodoptera frugiperda (J. E. Smith, 1797), has evolved resistance to several insecticides and Bacillus thuringiensis Berliner (Bt) toxins expressed in transgenic plants worldwide. Recently, we demonstrated a high risk of resistance evolution of a laboratory-selected emamectin benzoate-resistant strain of FAW based on incomplete dominant inheritance. In this study, we presented laboratory studies that established the baseline susceptibility and defined a diagnostic concentration of emamectin benzoate for the FAW performed in 2002. Then, resistance to emamectin benzoate was monitored in field-collected populations of FAW in Brazil from 2003 to 2004 and 2019 to 2021 using different methods: (i) estimating concentration-mortality curves; (ii) determining larval survival at a diagnostic concentration of emamectin benzoate; and (iii) assessing its survivorship on maize leaves sprayed with the field rate of this insecticide. Populations of FAW presented similar susceptibility to emamectin benzoate in 2002 (LC50 from 0.25 to 0.44 mu g a.i./mL) and 2003/04 (LC50 from 0.13 to 0.43 mu g a.i./mL), not differing from the susceptible reference population (LC50 of 0.22 mu g a.i./mL). From 2003 to 2004, larval survival at the diagnostic concentration of emamectin benzoate of 3.2 mu g a.i./mL was <1.79%. From 2019 to 2021, the susceptibility to emamectin benzoate in field populations of FAW decreased, with LC50 values ranging from 0.78 to 158.0 mu g a.i./mL, indicating resistance ratios up to 632-fold. Larval survival at a diagnostic concentration of emamectin benzoate also increased, reaching up to 81.4%. Larval survival on maize leaves sprayed with the field rate of emamectin benzoate ranged from 16.6 to 88.5% in populations of FAW collected in 2021. Overall, our findings revealed significant shifts towards decrease in the susceptibility to emamectin benzoate in Brazilian populations of FAW, providing robust evidence of field-evolved resistance.