学习英语的好文章.docx

1、学习英语的好文章 it was found that 5 per cent DDT powder caused no increase in their natural mortality rate. Similar results among lice collected from vagrants游民；流浪者 in Tokyo, from an asylum in Itabashi, and from refugee camps in Syria, Jordan, and eastern Egypt, confirmed the ineffectiveness of DDT for the

2、 control of lice and typhus. When by 1957 the list of countries in which lice had become resistant to DDT was extended to include Iran, Turkey, Ethiopia, West Africa, South Africa, Peru, Chile, France, Yugoslavia, Afghanistan, Uganda, Mexico, and Tanganyika, the initial triumph in Italy seemed dim i

3、ndeed. The first malaria mosquito to develop resistance to DDT was Anopheles sacharovi in Greece. Extensive spraying was begun in 1946 with early success; by 1949, however, observers noticed that adult mosquitoes were resting in large numbers under road bridges, although they were absent from houses

4、 and stables that had been treated. Soon this habit of outside resting was extended to caves, outbuildings, and culverts阴沟 and to the foliage and trunks of orange trees. Apparently the adult mosquitoes had become sufficiently tolerant of DDT to escape from sprayed buildings and rest and recover in t

5、he open. A few months later they were able to remain in houses, where they were found resting on treated walls. This was a portent预示；迹象 of the extremely serious situation that has now developed. Resistance to insecticides by mosquitoes of the anophelene group has surged upward at an astounding rate,

6、 being created by the thoroughness of the very house-spraying programs designed to eliminate malaria. In 1956, only 5 species of these mosquitoes displayed resistance; by early 1960 the number had risen from 5 to 28! The number includes very dangerous malaria vectors in West Africa, the Middle East,

7、 Central America, Indonesia, and the eastern European region. Among other mosquitoes, including carriers of other diseases, the pattern is being repeated. A tropical mosquito that carries parasites responsible for such diseases as elephantiasis象皮病 has become strongly resistant in many parts of the w

8、orld. In some areas of the United States the mosquito vector of western equine encephalitis马脑炎 has developed resistance. An even more serious problem concerns the vector of yellow fever, for centuries one of the great plagues of the world. Insecticide-resistant strains of this mosquito have occurred

9、 in Southeast Asia and are now common in the Caribbean region. The consequences of resistance in terms of malaria and other diseases are indicated by reports from many parts of the world. An outbreak of yellow fever in Trinidad in 1954 followed failure to control the vector mosquito because of resis

10、tance. There has been a flare-up疾病的发作 of malaria in Indonesia and Iran. In Greece, Nigeria, and Liberia the mosquitoes continue to harbor and transmit the malaria parasite. A reduction of diarrheal腹泻的 disease achieved in Georgia through fly control was wiped out within about a year. The reduction in

11、 acute conjunctivitis in Egypt, also attained through temporary fly control, did not last beyond 1950. Less serious in terms of human health, but vexatious令人烦恼的；麻烦的 as man measures economic values, is the fact that salt-marsh mosquitoes in Florida also are showing resistance. Although these are not

12、vectors of disease, their presence in bloodthirsty swarms had rendered large areas of coastal Florida uninhabitable until controlof an uneasy and temporary naturewas established. But this was quickly lost. The ordinary house mosquito is here and there developing resistance, a fact that should give p

13、ause to many communities that now regularly arrange for wholesale spraying. This species is now resistant to several insecticides, among which is the almost universally used DDT, in Italy, Israel, Japan, France, and parts of the United States, including California, Ohio, New Jersey, and Massachusett

14、s. Ticks are another problem. The woodtic木虱, vector of spotted fever斑疹热, has recently developed resistance; in the brown dog tick the ability to escape a chemical death has long been thoroughly and widely established. This poses problems for human beings as well as for dogs. The brown dog tick is a

15、semitropical species and when it occurs as far north as New Jersey it must live over winter in heated buildings rather than out of doors. John C. Pallister of the American Museum of Natural History reported in the summer of 1959 that his department had been getting a number of calls from neighboring

16、 apartments on Central Park West. Every now and then, Mr. Pallister said, a whole apartment house gets infested with young ticks, and theyre hard to get rid of. A dog will pick up ticks in Central Park, and then the ticks lay eggs and they hatch in the apartment. They seem immune to DDT or chlordane

17、 or most of our modern sprays. It used to be very unusual to have ticks in New York City, but now theyre all over here and on Long Island, in Westchester and on up into Connecticut. Weve noticed this particularly in the past five or six years. The German cockroach throughout much of North America ha

18、s become resistant to chlordane, once the favorite weapon of exterminators根除者；（美）灭鼠药 who have now turned to the organic phosphates. However, the recent development of resistance to these insecticides confronts the exterminators with the problem of where to go next. Agencies concerned with vector-bor

19、ne disease are at present coping with their problems by switching from one insecticide to another as resistance develops. But this cannot go on indefinitely, despite the ingenuity of the chemists in supplying new materials. Dr. Brown has pointed out that we are traveling a one-way street. No one kno

20、ws how long the street is. If the dead end is reached before control of disease-carrying insects is achieved, our situation will indeed be critical. With insects that infest crops the story is the same. To the list of about a dozen agricultural insects showing resistance to the inorganic chemicals o

21、f an earlier era there is now added a host of others resistant to DDT, BHC, lindane, toxaphene, dieldrin, aldrin, and even to the phosphates from which so much was hoped. The total number of resistant species among crop-destroying insects had reached 65 in 1960. The first cases of DDT resistance amo

22、ng agricultural insects appeared in the United States in 1951, about six years after its first use. Perhaps the most troublesome situation concerns the codling moth, which is now resistant to DDT in practically all of the worlds apple-growing regions. Resistance in cabbage insects is creating anothe

23、r serious problem. Potato insects are escaping chemical control in many sections of the United States. Six species of cotton insects, along with an assortment of thrips蓟马, fruit moths, leaf hoppers叶蝉, caterpillars, mites, aphids, wireworms金针虫, and many others now are able to ignore the farmers assau

24、lt with chemical sprays. The chemical industry is perhaps understandably loath to face up to the unpleasant fact of resistance. Even in 1959, with more than 100 major insect species showing definite resistance to chemicals, one of the leading journals in the field of agricultural chemistry spoke of

25、real or imagined insect resistance. Yet hopefully as the industry may turn its face the other way, the problem simply does not go away, and it presents some unpleasant economic facts. One is that the cost of insect control by chemicals is increasing steadily. It is no longer possible to stockpile ma

26、terials well in advance; what today may be the most promising of insecticidal chemicals may be the dismal failure of tomorrow. The very substantial financial investment involved in backing and launching an insecticide may be swept away as the insects prove once more that the effective approach to na

27、ture is not through brute force. And however rapidly technology may invent new uses for insecticides and new ways of applying them, it is likely to find the insects keeping a lap一圈 ahead. Darwin himself could scarcely have found a better example of the operation of natural selection than is provided

28、 by the way the mechanism of resistance operates. Out of an original population, the members of which vary greatly in qualities of structure, behavior, or physiology, it is the tough insects that survive chemical attack. Spraying kills off the weaklings. The only survivors are insects that have some

29、 inherent quality that allows them to escape harm. These are the parents of the new generation, which, by simple inheritance, possesses all the qualities of toughness inherent in its forebears. Inevitably it follows that intensive spraying with powerful chemicals only makes worse the problem it is d

30、esigned to solve. After a few generations, instead of a mixed population of strong and weak insects, there results a population consisting entirely of tough, resistant strains. The means by which insects resist chemicals probably vary and as yet are not thoroughly understood. Some of the insects tha

31、t defy chemical control are thought to be aided by a structural advantage, hut there seems to be little actual proof of this. That immunity exists in some strains is clear, however, from observations like those of Dr. Briejr, who reports watching flies at the Pest Control Institute at Springforbi, D

32、enmark, disporting娱乐；玩耍 themselves in DDT as much at home as primitive sorcerers魔术师 cavorting腾跃；欢跃 over red-hot coals. Similar reports come from other parts of the world. In Malaya, at Kuala Lumpur, mosquitoes at first reacted to DDT by leaving the treated interiors. As resistance developed, however

33、, they could be found at rest on surfaces where the deposit of DDT beneath them was clearly visible by torchlight. And in an army camp in southern Taiwan samples of resistant bedbugs were found actually carrying a deposit of DDT powder on their bodies. When these bedbugs were experimentally placed in cloth impregnated with DDT, they lived for a