Suscetibilidade a inseticidas em linhagens de Spodoptera frugiperda (J. E. Smith, 1797) com resistência as proteínas de Bacillus thuringiensis berliner expressas em milho

Abstract

Fall armyworm, Spodoptera frugiperda (J. E. Smith, 1797) (Lepidoptera: Noctuidae) is the most important pest of maize in Brazil. The control of this species has been performed with Bt maize and insecticides. However, the resistance evolution in S. frugiperda populations to Bt proteins and insecticides has threatened the sustainability of these control tactics. To support the management os this species, studies with resistant and susceptible strains of S. frugiperda were carried out in Bt and non-Bt maize technologies to evaluate the effectiveness of two seed treatments (chlorantraniliprole and imidacloprid + thiodicarb) and also its susceptibility to espinetoram and chlorfenapyr in diet-overlay bioassays (laboratory) and foliar sprays (greenhouse and field). S. frugiperda strains resistant to Cry1F, Cry1A.105 + Cry2Ab2 and Cry1A.105 + Cry1F + Cry2Ab2 were selected. In addition, a susceptible reference strain (Sus) and heterozygous obtained from reciprocal crosses were also evaluated were used in the experiments. In the first study, in laboratory leaf discs were obtained from maize plants whose seeds were treated with chlorantraniliprole or imidacloprid + thiodicarb. At 7, 14 and 21 days after emergence (DAE), in leaf discs from plants with seed treatment, the survival of resistant, heterozygous and susceptible strains was reduced up to 23.2; 24.8 and 28.2%, respectively, when compared to the same maize hybrid without seed treatment. Under field conditions, the same seed treatments showed low efficacy against natural infestations of S. frugiperda. Only at 7 DAE there was a significant reduction in the number of plants with damage caused by S. frugiperda. In the second study, the same strains were used in bioassays to evaluate the susceptibility to insecticides applied in the diet surface and plants. In the laboratory, larvae were grown on Bt and not Bt maize until the third larval instar when they were exposed to the insecticides spinetoram and chlorfenapyr in diet-overlay bioassays. Resistant and heterozygous strains showed similar susceptibility to spinetoram (LC50 = 0.16 to 0.18 ug a.i./cm2 ) and chlorfenapyr (LC50 = 0.17 to 0.20 ug a.i./cm2 ), when developed on Bt and non-Bt maize. However, Sus strain was more susceptible to both insecticides when fed on non-Bt maize; LC50 = 0.05 (spinoteram) and 0.08 (chlorfenapyr) ug a.i./cm2 . However, in greenhouse and field studies, no significant differences were detected in the survival of S. frugiperda when the commercial dose of both insecticides was applied in Bt and non-Bt maize. The results demonstrate that maize seed treatments with chlorantraniliprole or imidacloprid + thiodicarb have low control efficacy of S. frugiperda. It was also observed that strains of S. frugiperda fed on Bt and non-Bt maize have similar susceptibility to spinetoram and chlorfenapyr.