Field Crop 2024, Vol.7, No.5, 270-277 http://cropscipublisher.com/index.php/fc 272 field-evolved resistance to Bt cotton expressing Cry1Ac and Cry2Ab toxins has been documented, with resistance ratios (RRs) increasing significantly over time (Naik et al., 2018). The genetic basis of this resistance often involves mutations in the cadherin gene, which is crucial for the binding of Bt toxins (Fabrick et al., 2014). In China, similar resistance mechanisms have been observed, with mutations in the cadherin gene leading to resistance (Wang et al., 2019). The cotton bollworm, on the other hand, has been managed effectively in China through the use of natural refuges and hybrid seed strategies, which have delayed resistance development. 3.2 Factors contributing to resistance development in pests Several factors contribute to the development of resistance in cotton pests. One major factor is the lack of sufficient non-Bt cotton refuges, which are essential for maintaining a population of susceptible pests that can dilute the resistance genes (Quan and Wu, 2023). In India, the scarcity of non-Bt cotton refuges has been a significant issue, leading to widespread resistance in pink bollworm populations (Tabashnik and Carrière, 2019). Another factor is the genetic makeup of the pests themselves. Mutations in specific genes, such as the cadherin gene in pink bollworm, have been identified as key contributors to resistance (Wang et al., 2022). Additionally, the use of single-toxin Bt cotton varieties has been less effective in managing resistance compared to pyramided varieties that express multiple toxins (Fabrick et al., 2023). 3.3 Case examples of resistance emergence in different regions The emergence of resistance in cotton pests has been documented in various regions, each with unique contributing factors and outcomes. In the southwestern United States, a combination of non-Bt cotton refuges, sterile moth releases, and other integrated pest management tactics successfully delayed resistance and eventually eradicated the pink bollworm (Tabashnik et al., 2012). In China, the use of hybrid seeds that produce a mix of Bt and non-Bt cotton plants has been effective in managing resistance, with no reported failures in pest control over 25 years (Wan et al., 2017). However, in India, the lack of non-Bt refuges and the widespread planting of Bt cotton expressing Cry1Ac and Cry2Ab toxins have led to significant resistance issues, necessitating the adoption of integrated pest management strategies to mitigate the problem. 4 Current Resistance Management Strategies 4.1 Use of refuge strategies in Bt cotton cultivation Refuge strategies are a cornerstone in managing resistance to Bt cotton. These strategies involve planting non-Bt crops alongside Bt crops to provide a habitat for susceptible pests, thereby diluting the population of resistant pests. In the United States, the high-dose/refuge strategy has been particularly effective. This approach involves planting non-Bt cotton refuges to delay resistance, as seen in the successful management of pink bollworm in the southwestern United States from 1996 to 2005 (Huang et al., 2011). Similarly, in China, a seed mix refuge strategy, which includes a 25% non-Bt cotton mix, has been employed to manage resistance in pink bollworm effectively (Figure 2) (Quan and Wu, 2023). The natural refuge strategy, involving non-Bt crops like corn, soybeans, and peanuts, has also been used to manage resistance in polyphagous pests such as the cotton bollworm (Arends et al., 2021). 4.2 Integrated pest management (IPM) practices alongside Bt cotton Integrated pest management (IPM) practices are essential in complementing Bt cotton to manage pest resistance. IPM involves a combination of biological, cultural, and chemical control methods to manage pest populations sustainably. In India, IPM practices such as shortening the cotton season and destroying crop residues have become crucial due to widespread resistance to pyramided Bt cotton (Tabashnik and Carrière, 2019). In the United States, IPM practices have been recommended to prolong the durability of Bt technologies, especially for pests like the western corn rootworm (Martinez and Caprio, 2016). The use of IPM has also been shown to enhance biological control, contributing to the suppression of other key and sporadic pests in cotton (Naranjo, 2011). 4.3 Benefits and limitations of current management strategies The current resistance management strategies offer several benefits but also come with limitations. The high-dose/refuge strategy has been successful in delaying resistance and maintaining the efficacy of Bt crops in various regions, including North America and Australia (Knight et al., 2021). The use of natural refuges and seed
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