Susceptibilidade de uma população de Spodoptera frugiperda (Lepidoptera: Noctuidae) ao metomil por sucessivas gerações

Abstract

Maize is a crop of major economic importance; however, its productivity is affected by insect pests, especially Spodoptera frugiperda (J.E. Smith, 1797) (Lepidoptera: Noctuidae), which is considered the most important pest of this crop. Control is primarily achieved through the use of chemical insecticides based on methomyl, whose continuous application may promote the development of resistance. In this context, the objective of this study was to evaluate the susceptibility of S. frugiperda to the insecticide methomyl over successive generations, as well as to analyze the dynamics of resistance development and its possible reversion following the interruption of selection pressure. To this end, field-collected individuals were maintained under laboratory conditions (25 ± 2°C, 70 ± 10% RH, and a 12-h photophase). Bioassays were conducted using third-instar larvae exposed to methomyl applied to maize leaf discs treated with different insecticide concentrations. Applications were performed for 11 consecutive generations, suspended for two generations, and resumed in the 14th generation. Mortality was assessed after 72 hours of exposure, and the data were subjected to Probit analysis to determine the median lethal concentration (LC₅₀). Resistance ratios (RR) were also calculated in comparison with a susceptible reference population. The results showed a progressive increase in resistance up to generation 11, when the highest LC₅₀ value (101.96 μg a.i. mL⁻¹) and a resistance ratio of 4.80 were recorded, characterizing a low level of resistance. Following the interruption of insecticide application, a reduction in both LC₅₀ and resistance ratio was observed in generation 14. The susceptible population presented an LC₅₀ of 16.97 μg a.i. mL⁻¹, whereas the methomyl-selected population (G14) exhibited an LC₅₀ of 37.06 μg a.i. mL⁻¹ and a resistance ratio of 2.18, indicating reduced susceptibility to the insecticide and partial resistance reversion in the absence of selection pressure. It can be concluded that resistance of S. frugiperda to methomyl exhibits a dynamic behavior that is dependent on selection pressure. Therefore, the adoption of strategies such as insecticide rotation and integrated pest management is essential to preserve the effectiveness of chemical control.