Genetically modified (GM) cotton expressing Bacillus thuringiensis ( Bt ) toxins is an important tool for managing lepidopteran pests in Brazil; however, its efficacy against Spodoptera frugiperda (J. E. Smith) is reduced because of widespread resistance to Bt maize. Here, we evaluated the performance of Bt cotton and insecticides against Vip3Aa20-resistant, Vip3Aa20-susceptible, and heterozygous strains. The study included two phases: (i) assessment of larval survival and development on Cry1Ac/Cry2Ab2/Vip3Aa20, Cry1Ac/Cry2Ab2, and non- Bt cotton, and (ii) evaluation of the efficacy of thiodicarb and flubendiamide against L2 and L4 larvae surviving on these cotton types. Survival patterns on Bt cotton differed among strains: resistant larvae reached adulthood on both Bt cotton types, albeit with prolonged developmental time; heterozygous larvae survived only on Cry1Ac/Cry2Ab2 cotton; and susceptible larvae died in early instars on both Bt technologies. All strains showed high survival on non- Bt cotton. Thus, Cry1Ac/Cry2Ab2/Vip3Aa20 cotton controlled susceptible and heterozygous larvae completely but allowed substantial survival of resistant larvae, whereas Cry1Ac/Cry2Ab2 cotton was effective mainly against susceptible insects. Thiodicarb caused consistently high mortality of L2 and L4 larvae, including Bt and non- Bt survivors, whereas flubendiamide showed limited lethality. Although combining either type of Bt cotton with thiodicarb improved overall control, it did not fully suppress resistant larvae. These results demonstrate that Vip3Aa20 resistance in S. frugiperda reduces the efficacy of current Bt cotton technologies and underscore the need to integrate Bt cotton with insecticides for the management of this pest. • Vip3Aa20 resistance in Spodoptera frugiperda reduces Bt cotton efficacy. • Cry1Ac/Cry2Ab2/Vip3Aa20 cotton controls susceptible and heterozygous, but not resistant larvae. • Cry1Ac/Cry2Ab2 cotton controls only susceptible larvae. • Thiodicarb controls Vip3Aa20-resistant larvae across instars in Bt and non- Bt cotton. • Flubendiamide shows reduced control of resistant and heterozygous in both cotton types.
Palharini et al. (Wed,) studied this question.