Bt Research 2025, Vol.16, No.4, 168-181 http://microbescipublisher.com/index.php/bt 175 important cotton-producing area in China. Cotton bollworms occur frequently in this area, and farmers previously needed to spray a lot of cotton to protect them. Since the promotion of Bt cotton in the late 1990s, the pest control pattern in cotton areas in the Yellow River Basin has undergone significant changes (Quan and Wu, 2023). Field surveys in cotton areas such as Dezhou and Liaocheng, Shandong showed that the field density of cotton bollworm larvae after introduction of Bt cotton was reduced by more than 80% compared with conventional cotton planting, which greatly reduced the number of cotton bollworm damage. Cotton farmers generally report that Bt cotton rarely "beaten insects". Most fields only spray medicines for secondary insects once during the intercropping period, and the plant insurance investment has been significantly reduced (Figure 2). In terms of output, taking Texas region as an example, after promoting Bt cotton, the yield per mu increased from about 80 kg per mu of conventional varieties to more than 110 kg. Due to the reduced pest pressure, the cotton breeding period is more complete, the rate of berry bell shedding decreases, and the number of peaches is significantly increased. Bt cotton planting in the Yellow River Basin not only improves cotton fields' income, but also brings ecological benefits. Reduced pesticide use in cotton fields promotes the recovery of natural enemy insect populations (Luo et al., 2014). In observations in Anyang, Henan and other places, the number of natural enemies such as spiders and grass flies in Bt cotton fields is several times higher than that in non-Bt cotton fields, and no serious outbreaks of secondary pests such as cotton aphids. This shows that Bt cotton helps maintain ecological balance in farmland. Figure 2 Natural refuge strategy (A) and field monitoring of Helicoverpa armigera regarding Cry1Ac resistance IC50s and alleles (B,C) in China. The IC50 is the concentration of the Cry1Ac protein that inhibits larval development by 50%, and the error bars stand for the 95% fiducial limits. The frequency of alleles in field Helicoverpa armigera was obtained through an F1 screen from the previous work (Adopted from Quan and Wu, 2023) 6 Resistance Management and Sustainable Utilization 6.1 Molecular mechanisms of pest resistance evolution Cases of pests resistant to Bt genetically modified crops have gradually increased worldwide in recent years. Research shows that resistance often originates from the fact that some individuals in the pest population carry gene variants that reduce their sensitivity to Bt toxins. These variants accumulate gradually under continuous selection pressure, ultimately leading to the emergence of resistant lines. From the perspective of molecular mechanisms, the resistance of different pests to Bt toxins can be caused by multiple pathways. Among them, receptor protein mutation is one of the most common mechanisms. When Bt toxin cannot find a binding target in the insect midgut, it will be difficult to exert poisoning. For example, the population of the American population
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