听力原文： Scientists have discovered that plants themselves pro-duce many poisonous chemicals for defense against insects. But insects have developed ways of dealing with them and, in fact, have been eating plants for about 250 million years. Now insects are using these same mechanisms to deal with man-made poisons--insecticides. This is why resistance of insects to insecticides has developed so rapidly. What should be done? Scientists studying the problem suggest that farmers use less insecticides. At the moment, farmers regularly spray crops as a precaution against problems that are caused by large numbers of pests. They should, instead, spray only where pests have actually been seen. Secondly, farmers could use a combination of two or three insecticides at once. To survive, the insects would need to become resistant to two or more insecticides at the same time. The most surprising suggestion, perhaps, is that at certain times of the year, farmers should actually try to at-tract insects onto the crops they are trying to protect. The new insects will mate with those which survived from a certain kind of insecticide and will lessen the latter's resistance to it. Scientist hope that these and other measures will post-pone the day when farmers and scientists will have to stand by while new super-bugs which may be resistant to all poi-sons invade our farms and devour our crops. What conclusion can be drawn from the scientists' observation?