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对于危害作物的某些害虫来说,化学药剂往往很少或没有效果。于是害虫不断繁殖,并损害更多的作物。侵袭树木的舞毒蛾,便是这样的一种害虫。大约100年以前,作为改良蚕种试验的一部分,这种蛾子从欧洲带到了美国。但蛾子逃跑了,由于缺乏天敌,不久便迅速遍布整个美国。从此以后,化学药剂对这种蛾子的控制失灵。科学家说,毒药对环境造成的危害,比对舞毒蛾造成的危害更大。不过,大约10年之前,美国科学家认为,他们发现了一种控制舞毒蛾的方法。
For some pests that endanger crops, chemicals often have little or no effect. As a result, pests continue to multiply and damage more crops. Gypsy moths that attack the trees are such pests. About 100 years ago, this moth was brought from Europe to the United States as part of a trial to improve silkworm eggs. But moths escaped and soon became widespread throughout the United States due to a lack of natural enemies. Since then, chemicals have failed to control this moth. Scientists say the harm done by poison to the environment is more harmful than to gypsy moth. However, about 10 years ago, U.S. scientists thought they found a way to control Gypsy moths.