Susceptibility of Cry1F-maize resistant, heterozygous, and susceptible Spodoptera frugiperda to Bt proteins used in the transgenic cotton

Fall armyworm, Spodoptera frugiperda U.E. Smith), is a major target species of Bt maize and Bt cotton in the United States. Field resistance to Cry1F maize in S. frugiperda has occurred in Puerto Rico, Brazil, and the southeast region of the United States. There is a great concern that the Cry1F-maize resistant S. frugiperda may move to the cotton fields and cause control problems to Bt cotton where both crops are planted. In this study, larval mortality and growth inhibition of Cryl F maize-susceptible (SS),-heterozygous (RS), and-resistant (RR) genotypes of S. frugiperda were evaluated using diet-incorporated bioassays with six Bt proteins that have been used in commercialized and future Bt cotton plants: Cry1F, Cry1Ab, Cry1Ac, Cry2Ab2, Cry2Ae, and Vip3A. Compared to the mortality data, the measuring of growth inhibition was more sensitive in determining Bt susceptibility of the insect to all six Bt proteins. Overall, RS had a similar performance as SS for the six Bt proteins with a resistance ratio of <5.5-fold. The resistance ratio of RR calculated based on larval growth inhibition was 930-fold for Cry1F, 50.5-fold for Cry1Ab, and >172-fold for Cry1Ac. In contrast, all three insect genotypes were susceptible to Cry2Ab2, Cry2Ae, or Vip3A. The results showed that the Cry1F-maize resistant S. frugiperda was cross-resistant to CrylAb or Cry1Ac, but not cross-resistant to Cry2Ab2, Cry2Ae, or Vip3A. The strong cross-resistance among Cry1 proteins suggest that it is necessary to plant pyramided cotton traits that contain at least two active Bt genes for Cryl F-resistant S. frugiperda to ensure the continued success of the Bt cotton technology, especially in the areas where Bt resistance has already occurred. (C) 2017 Elsevier Ltd. All rights reserved.