Bt Research 2024, Vol.15, No.1, 30-41 http://microbescipublisher.com/index.php/bt 33 rhizosphere, root endosphere, or leaf endosphere over multiple years (Wu et al., 2021). Overall, the evidence suggests that Bt transgenic crops do not pose significant risks to non-target organisms in agricultural systems (Mandal et al., 2020; Li et al., 2022). 4 Resistance Management 4.1 Development of pest resistance The widespread adoption of Bt transgenic crops has significantly improved pest management and reduced reliance on chemical insecticides. However, the evolution of pest resistance to Bt toxins poses a major challenge to the long-term sustainability of these crops. Over the past two decades, the number of cases of pest resistance to Bt crystal proteins (Cry proteins) has increased significantly, from three cases in 2005 to 16 cases in 2016 (Tabashnik and Carrière, 2017). Similarly, a comprehensive analysis of global resistance monitoring data over 25 years has shown substantial resistance in some pest populations to nine widely used Cry toxins, spanning 11 pests in seven countries (Tabashnik et al., 2023). Research indicates that the development of pest resistance primarily stems from mutations affecting Bt toxin activation, toxin-binding genes, and regulation of the pest immune system. These mechanisms enable pests to survive Bt crops, gradually accumulating resistance genes and spreading within populations, as seen in the evolution of resistance in pink bollworm (PBW) to Bt cotton (Figure 2). In the long term, the evolution of resistance may significantly reduce the efficacy of Bt crops. Therefore, understanding the mechanisms behind resistance development and implementing integrated management strategies are crucial (Xiao and Wu, 2019). Figure 2 Models of the evolution of pink bollworm (PBW) Bt resistance (Adopted from Xiao and Wu, 2019) Figure 2 illustrates the resistance evolution model of pink bollworm (PBW) to Bt cotton. In the United States, by effectively implementing a high-dose/refuge strategy, Bt cotton successfully delayed the spread of resistance genes, almost eliminating the resistant population after the introduction of multi-toxin Bt cotton. In India, poor refuge management led to the rapid development of Cry1Ac resistance in single-toxin Bt cotton, with resistance gradually increasing in subsequent multi-toxin Bt cotton. In China, the mixed refuge strategy with F2 generation
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