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 poisons—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 numbers 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.