第三篇文章 pest control
文章第一部分化学手段进行防治。但是化学手段存在缺点：但是由于害虫的繁殖速度快， 同时农药还大量杀死的predator, 但是predator 就没这么幸运了，他们不能大量繁殖，而且毒素可以传递到他们身体里，从而杀死prdator，这一点造成了害虫数量的增加。举了中国的例子，作为第一个采用化学防治的例子。
文章第二部分将采用天敌进行防治。引进的pest的天敌也许会成功，但是最后predator一段时间之后也可能成为另一种危害。举了california的例子说明。
文章第三步部分说明在用上述方法综合使用。首先先考虑化学方法，如果不能控制就引入天敌，但是只要能够控制在不造成经济危害的基础之上.
解析;文章重点为探讨两种对于害虫的控制方法，生物控制和化学控制，分别探讨各自的优缺点。最后提出二者的综合，提出操作的过程。大家按照不同方法的不同特点梳理文章的重点信息即可。
Chemical Control
Chemical pesticides are substances that are manufactured in laboratories that, when applied to crops, reduce the vitality of pest populations while leaving crops unharmed. There are many chemicals available to help eradicate common pests in a number of ways. Chemical controls can kill pests that come in contact with the chemical (toxicants), eliminate the reproductive potential of pests (sterilants), disrupt their developmental potential (growth regulators) or influence their behavior (semiochemicals). Most of these chemical controls are fast acting and effective.
Biological Control
Biological control methods employ the use of living organisms such as predators, parasites and pathogens to control the populations of pests on agricultural crops. Biological control agents can be bred and reared in large numbers and then released into infected crops to reduce the populations of pests (augmentation) or simple land conservation measures can be implemented on agricultural lands that maintain healthy populations of native predators (conservation). Many pests that cause damages to crops thrive because they are invasive and have no natural predators. Finding and importing predators of these invasive pests is essential for effective biological pest control.
Benefits
Chemical controls are cheap and readily available. Chemical controls, especially toxicants, have been in use since the 1940's and have remained in popular use due to their fast acting and effective results in controlling pest populations. Many new chemicals have been developed in recent years that are even more efficient in controlling pests, maintaining the popularity of chemical control in agricultural practices. However, biological control has seen an increase in use in recent years due to its perennial and organic nature. Many biological control methods remain in effect year after year, limiting pests without any additional costs or synthetic additives to the natural environment.
Considerations
While chemical controls are often effective they are usually seasonal and require reapplication with each growing season. Biological controls may take a longer period of time to see the desired results, but they only require the initial investment and introduction to control pests. Chemical controls also have additional environmental costs. Many chemical pesticides are persistent in the environment, damage organisms other than the pests they are meant to control (including humans) and are not permanently effective, as pest populations can build up a resistance to chemicals over time. Thus, while chemical controls may be more economical and effective in the short term, their use requires caution and consideration for future costs, both environmental and economic.
Integration
While some landowners look only at seasonal profits and depend on chemical methods, others contemplate only the environmental sustainability of their practices and opt for biological methods. However, many landowners blend chemical and biological controls together in order to maximize profits while minimizing costs as well as reduce the environmental impact on their land. The use of multiple pest control methods is referred to as integrated pest management (IPM). Dense infestations often require the potency of chemical pest control but limited application, coupled with preventative biological control, is the most effective agricultural management practice.
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