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托福阅读材料之处理害虫

2015-01-06 16:24:12来源：网络

　　托福阅读偏重于有一些专业性并且有一些难度的文章，在托福阅读练习材料选择上要注意。那么我们在进行托福阅读练习的时候，应该选择什么样的托福阅读资料呢?本期为大家分享托福阅读材料之处理害虫，帮助大家备考复习。

　　Insects become resistant to chemical insecticides very rapidly—it can happen in as few as five generations. This is natural selection at work.

　　The problem is that an insecticide never kills all of its intended victims. If even a few insects survive, they will reproduce. These surviving insects will produce two types of young—those that are resistant to the spray, and those that are not. The non-resistant insects will be killed in the next spraying, but those that are left reproduce. At each generation, the number of naturally resistant insects in the population increases.

　　An individual insect does not become resistant during its lifetime. It is born either resistant or non-resistant, and it is the population as a whole that gradually becomes resistant to the pesticide over time. The Bt toxins become ineffective, and the benefits of using them (less toxicity to non-target species) disappear.

　　As this occurs, a new pesticide must be developed. Over time, populations of insects can become resistant to more and more pesticides. As a result, humans need to make different pesticides that are generally stronger.

　　Organic farmers have used Bt on their crops for a number of years. They are concerned that the increased use of the Bt toxin could speed up the development of resistant insect populations.

　　Entomologists know that controlled, laboratory experiments with generations of insects cannot be easily reproduced in the field. How the resistant insects breed with refuge insects, and over what time frames, will determine the success of this technology.

　　These concerns are balanced by concerns that existing pesticide practices can be much more dangerous for non-target insect species than insect-resistant crops. Conventional non-selective pesticides kill many non-target insects. By reducing the number of sprays needed, insect-resistant crops help to preserve beneficial predator insects and simplify management decisions.

　　以上就是新东方在线托福小编整理的托福阅读的相关内容，更多托福考试分析敬请访问新东方在线托福频道，祝大家都取得理想的成绩!


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