在以往的托福阅读考试中有关于害虫处理的考试文章。针对这一考试话题，前程百利小编为大家普及一下相关背景知识，这样有助于考生在面对这类题目时方便作答。托福阅读真题文章讲解了处理害虫的不同方法。理解文章时需要按照不同的处理方法梳理文章的结构，要把握住作者关于处理方法优缺点的介绍，不同方法的不同特点为文章出题点。
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.
以上就是今天的托福阅读材料，大家可以在练习托福阅读材料的同时，积累一些相关词汇及句型，以便考试的时候更好地把握文意，夺得托福阅读的高分。前程百利小编预祝大家托福考试取得优异成绩!
