In recent years, farmers in the Central South China Cotton Region have reported that insect-resistant cotton is not as effective as expected in controlling pests. This has raised questions about whether Bt cotton is truly resistant to insects. The answer is no—it's not completely pest-free. Understanding the characteristics of insect-resistant cotton is essential for farmers to implement proper management strategies and prevent potential damage.
First, it's important to understand the insect-resistance mechanism. Bt cotton contains a transgenic gene from *Bacillus thuringiensis* that produces toxins specifically targeting Lepidoptera pests like the cotton bollworm. While this makes it highly effective against certain pests, it does not provide broad-spectrum protection. Under normal conditions, the resistance genes function well, but they are not foolproof. Farmers must still monitor fields and apply control measures when necessary, as even Bt cotton can suffer from pest infestations if not managed properly.
The national standards for assessing insect-resistant cotton include several key criteria. One is the toxic protein content: samples taken from the second and third true leaves of cotton plants should have at least 200 ng/g, with over 400 ng/g considered high resistance. Another measure is the reduction in bollworm damage—cotton with a 50-60% reduction is considered low resistance, while 60-80% is high. In lab tests, larvae exposed to Bt cotton must show a mortality rate of over 80% to be classified as high resistance. Field resistance rates also matter—over 90% resistance is required for certification, and more than 95% indicates very strong resistance.
Despite these standards, resistance can weaken due to various factors. For example, genetic instability in hybrid varieties can lead to non-resistant plants appearing in the field. Over time, if varieties are not properly maintained or rejuvenated, their resistance may decline. Environmental stressors such as drought, high temperatures, or poor soil nutrients can also interfere with gene expression, reducing effectiveness.
Additionally, different parts of the cotton plant have varying levels of resistance. The tender tips and young leaves are most resistant, while buds and flowers are more vulnerable. This means that eggs laid on buds or flowers may hatch into larvae that survive, causing damage. Farmers should pay extra attention to these areas during monitoring.
Another critical factor is the development of resistance in pests. Due to long-term use of Bt cotton, some populations of *Helicoverpa armigera* have adapted, reducing the effectiveness of Bt toxins. This natural selection process highlights the need for integrated pest management.
In conclusion, Bt cotton is not a magic solution. It still requires regular field inspections and timely intervention. If thresholds are met—such as more than 20 eggs per 100 plants or 100 third-instar larvae—immediate action is necessary. Prevention should occur three days after peak egg-laying, and multiple applications may be needed depending on the generation. Pesticide use should be carefully managed, avoiding over-reliance on a single type and rotating products to maintain efficacy. Spraying should be done early in the morning or late in the afternoon, with careful, uniform coverage to ensure optimal results.
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