TEXT C 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. According to Fields, all of the following are the advantages of his technique EXCEPT that ______.
A．it is environmentally friendly B．it is safe to the people’s health C．it is more effective than pesticides against some insects D．it is cheaper than pesticides