由于 Bt棉第一代和第二代品种对棉铃虫有高度的抗性 ,基本不再对棉铃虫采取化学用药防治 ,使棉田用药量减少了 80 %以上 ,棉田天敌种群大幅度上升 ,促进了棉田生态结构的变化 ,使益害比向生态平衡方向发展 . Because the first and second generations of Bt cotton were highly resistant to the cotton bollworm, chemical control was no longer applied to the cotton bollworm and the dosage of cotton field was reduced by more than 80%. The natural enemies of cotton fields increased significantly and promoted Changes in the ecological structure of the cotton field, so that harm than harm to the ecological balance direction.