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Pesticides are chemicals that farmers use to kill insects that damage their crops. Many of them work by attacking an insect's nervous system, so that the insect can no longer move or feed. When the first modern pesticides came into wide use in the 1940s, the results seemed almost magical. Crops that had once been destroyed every year by insects could now be protected, and harvests increased dramatically.
However, the victory did not last. In any large population of insects, there are almost always a few individuals that, by chance, are not affected by a particular chemical. They may have a slightly different body chemistry that breaks down the poison, or a thicker outer layer that keeps it out. For these few insects, the pesticide is not a threat but an advantage. It kills their competitors and leaves them with more food and space. They survive, they reproduce, and their offspring inherit the same resistance.
Within a few generations, which for many insects means only a few years, most of the population may be resistant. This process happened surprisingly fast. By the late 1940s, only a few years after the pesticide DDT came into wide use, scientists had already reported houseflies that it could no longer kill.
For a long time, farmers dealt with resistance by spraying more often or by switching to a new chemical. If one pesticide stopped working, another one usually did. But as time passed, more and more insect species became resistant to several pesticides at once. Today, hundreds of species of insects are known to resist at least one pesticide, and some resist almost all of the chemicals that farmers can buy.
The appearance of resistant insects should not have been a surprise. Nevertheless, for many years there was a sense of confidence, even carelessness, about the problem. Chemical companies found it more profitable to sell existing products than to develop new ones, and few people thought about the long-term consequences. As a result, the development of new pesticides fell behind the spread of resistance.
In recent years, scientists and farmers have begun to look for different approaches. One is to rotate crops and chemicals, so that insects are not exposed to the same poison year after year. Another is to use natural enemies, such as birds, spiders and certain wasps, which feed on harmful insects. A third is to plant crops that are naturally less attractive to pests. Most experts now agree that no single method will solve the problem, and that farmers will need to combine several approaches if they want to stay one step ahead of the insects.