填空题

Scientists have discovered that plants themselves produce many poisonous chemicals for (36) against insects. But insects have developed ways of (37) with them and, in fact, have been eating plants for about 250 million years. Now insects are using these same (38) to deal with man-made poi sons--insecticides. This is why (39) of insects to insecticides has developed so (40) What should be done Scientists studying the problem suggest that farmers use less insecticides. At the moment, farmers (41) spray crops as a (42) against problems that are caused by large number of pests. They should, instead, spray only where pests have actually been seen. Secondly, farmers could use a (43) of two or three insecticides at once. (44) The most surprising suggestion, perhaps, is that at certain times of the year, (45) . The new insects will mate with those which survived from a certain kind of insecticide and will lessen the latter’s resistance to it. (46) which may be resistant to all poisons invade our farms and devour our crops.