四级冲刺练习阅读169

　　For farmers like Anderson, pesticide use has become not the solution but a cause of many pest problems. Rachel Carson predicted as much 30 years ago in Silent Spring, though the public paid little notice amid the furor her book sparked over pesticides ecological and health effects. In recent years, however, pesticides shortcomings have grown harder to ignore in light of mountain pesticide resistance and destruction of beneficial insects. In fact, a growing number of agricultural experts now argue that reducing pesticide use can actually decrease pests. Pest control has reached a turning point, says pest control expert Robert Metcalf of the University of Illinois at Urbana.

　　When DDT, the first widely used synthetic pesticide, hit the market in 1946, it looked like the silver bullet that would wipe out insect pests forever. Before DDT, American farmers lost about a third of their crops each year to insects, weeds and disease. Today, with an annual pesticide bill exceeding $ 4 billion, farmers still lose the same one-third sharea loss that mounts into the tens of billions of dollars each year.

　　Chemical pest control has grown steadily more difficult because of a growing number of pesticide-resistant insects and weeds. Resistance is biologically inescapable: Each time a farmer sprays a field, the few bugs genetically able to tolerate the poison stand the best chance of surviving to produce the .next generation of increasingly resilient insects. Its just accelerated evolution. Darwin would be pleased, says Metcalf. In 1948 just 14 species of insects were resistant to one or more pesticides; more than 500 are resistant today. Even the bacterial insecticide Bacillus thuringiensis, or Bt, once touted as practically resistance proof because of its complexity, is beginning to lose its effectiveness on a few agricultural pests.

　　More ominous , several important insects have developed resistance to every major insecticide. In the state of Gujarat in India, for instance, the mosquitoes that transmit malaria are resistant to every affordable insecticide, and malaria rates are surging. Similarly in the northeastern United States, the Colorado potato beetle has become resistant to at least 15 chemicals, leaving potato growers dependent on a compound not yet formally approved for potatoes.

　　Pesticides also create new pests because they destroy the spiders, wasps and predatory beetle that naturally keep most plant-feeding insect populations in check. The brown plant hopper that plagued Indonesian rice fields in the 1970s and 80s was not a serious problem until 1970, shortly after heavy insecticide use began. In the United States, such major pests as spider mites and cotton bollworm were nuisances at most until spraying decimated their