In recent years, farmers in the Central South China Cotton Region have reported that insect-resistant cotton is not as effective as expected against pests. This has raised questions about whether Bt cotton is truly resistant to insects. The answer is no—while it offers protection, it is not completely pest-free. Understanding its characteristics is crucial for implementing effective management strategies.
First, the insect resistance of Bt cotton comes from the transgenic Bt gene, which targets specific pests, mainly Lepidoptera such as the cotton bollworm. It works by disrupting the feeding and growth of these pests, reducing their population. Under normal conditions, the gene functions well, offering strong resistance. However, Bt cotton is not a solution that eliminates all pests. Field monitoring is still necessary, as improper management can lead to infestations.
The national standard for Bt cotton includes several criteria to determine its resistance level. These include the concentration of toxic proteins in the leaves, the reduction in bollworm damage compared to conventional cotton, the mortality rate of larvae in lab tests, and the field resistance rate. According to these standards, cotton with more than 200 ppm of toxic protein is considered resistant, while over 400 ppm indicates high resistance. Similarly, a 50-60% reduction in damage marks moderate resistance, and above 80% shows high resistance. These metrics highlight that resistance is relative and depends on multiple factors.
Poor resistance in some Bt cotton varieties can occur due to various reasons. First, genetic instability may arise from hybrid breeding, leading to plants that are not fully resistant. Additionally, natural variations and environmental stressors like drought or high temperatures can reduce the expression of the Bt gene. Over time, if the variety is not properly maintained, it can degrade, further lowering its effectiveness.
Different parts of the cotton plant also show varying levels of resistance. The tender tips are most resistant, followed by young leaves, older leaves, buds, and finally flowers. Since buds and flowers are less resistant, they can become sites for bollworm eggs, allowing larvae to survive and cause damage.
Another factor is the "natural law" of evolution. Prolonged use of Bt cotton can lead to the development of resistant Helicoverpa armigera populations, reducing the effectiveness of the technology over time. This means that even Bt cotton may eventually lose its potency without proper management.
In conclusion, Bt cotton is not a substitute for regular monitoring and control. Farmers should inspect fields regularly and act when thresholds are reached. For example, removing non-resistant plants can prevent pests from spreading. The best time to control bollworms is within three days after egg-laying peaks, and timely application of pesticides is essential. Using a single pesticide repeatedly can lead to resistance, so alternating products is recommended. Always follow label instructions and spray during optimal times—before 10 am or after 5 pm. Ensure even coverage to maximize effectiveness. With proper care, Bt cotton can remain a valuable tool in sustainable pest management.
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