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  3. Insects which eat grain and other stored food can be partially controlled by cooling the grain. But the damage insects cause can

Insects which eat grain and other stored food can be partially controlled by cooling the grain. But the damage insects cause can

admin2013-11-10  42
问题      Insects which eat grain and other stored food can be partially controlled by cooling the grain. But the damage insects cause can be further limited by making them even more susceptible to cold, says a Canadian researcher.
     Insect pests die if they freeze, but many have the ability to become "supercool", remaining alive even when the temperature drops to - 10℃. However, Paul Fields of the Agriculture Canada Research Station in Winnipeg has discovered that the ability of these insects to supercool can be seriously affected by "ice-nucleating-active bacteria". When the bacteria are present, the insects freeze and die at a higher temperature than normal.
     Ice-nucleating-active bacteria are found naturally on most plant leaves. They are also grown commercially for use in cloud seeding and in making artificial snow.
     Fields added these bacteria to wheat which was stored at a temperature of - 10℃ and contained the rusty grain beetle. He found that when the bacteria accounted for only 10 parts per million of the grain, the beetles’ supercooling point rose from - 11.3℃ to - 7.1℃, so more died. At concentrations of 100 ppm or 1000 ppm, the bacteria increased the supercooling point to as high as -6.3℃, which resulted in more than 75 percent of the beetles dying.
     Fields also applied the bacteria to two other species of grain beetle, Sitophilus granarius and Oryzaephilus surinamensis, keeping them at - 10℃. He found that more of these beetles also died. First, though, he gradually acclimatized some of the insects to cold, to simulate the increase in tolerance to cold which they normally develop as winter approaches. Fields found that the insects that had been acclimatized were less affected by cold than the beetles that had not been acclimatized, but that more of them still died at - 10℃.
     Fields says that the technique of using ice-nucleating-active bacteria leaves no noxious chemical residues and poses little risk to the people who apply the treatment. It is also effective against insects which have become resistant to pesticides.
Fields found that when the bacteria accounted for 100 parts per million of the grain, the beetles’ supercooling point ______.
选项 A、rose by 4℃
B、dropped by 4℃
C、rose by 5℃
D、dropped by 5℃
答案C
解析 第四段说当谷物中这种细菌的浓度达到100ppm或1000ppm的时候，会使虫子的抗低温能力由-11.3℃减弱为-6.3℃，因此选C。
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