GRE·94-10考题—（6）

文章作者 100test 发表时间 2007:03:25 21:36:38
来源 100Test.Com百考试题网

The defoliation of millions of acres
　　of trees by massive infestations of
　　gypsy moth caterpillars is a recurring
　　phenomenon in the northeastern United
(5) States. In studying these outbreaks,
　　scientists have discovered that
　　affected trees fight back by releasing
　　toxic chemicals, mainly phenols, into
　　their foliage. These noxious substances
(10) limit caterpillars growth and reduce
　　the number of eggs that female moths
　　lay. Phenols also make the eggs
　　smaller, which reduces the growth of
　　the following year s caterpillars.
(15) Because the number of eggs a female
　　moth produces is directly related to
　　her size, and because her size is deter-
　　mined entirely by her feeding success as
　　a caterpillar, the trees defensive
(20) mechanism has an impact on moth
　　fecundity.
　　　The gypsy moth is also subject to
　　attack by the nucleo-polyhedrosis
　　virus, or wilt disease, a particularly
(25) important killer of the caterpillars in
　　outbreak years. Caterpillars contract
　　wilt disease when they eat a leaf to
　　which the virus, encased in a protein
　　globule, has become attached. Once
(30) ingested by a caterpillar, the protein
　　globule dissolves, releasing thousands
　　of viruses, or virions, that after
　　about two weeks multiply enough to fill
　　the entire body cavity. When the
(35) caterpillar dies, the virions are
　　released to the outside, encased in a
　　new protein globule synthesized from
　　the caterpillar s tissues and ready to
　　be picked up by other caterpillars.
(40) 　Knowing that phenols, including
　　tannins, often act by associating with
　　and altering the activity of proteins,
　　researchers focused on the effects on
　　caterpillars of ingesting the virus and
(45) leaves together. They found that on
　　tannin-rich oak leaves, the virus is
　　considerably less effective at killing
　　caterpillars than when it is on aspen
　　leaves, which are lower in phenols.
(50) In general, the more concentrated
　　the phenols in tree leaves, the less
　　deadly the virus. Thus, while highly
　　concentrated phenols in tree leaves
　　reduce the caterpillar population by
(55) limiting the

size of caterpillars and
　　consequently, the size of the female s
　　egg cluster, these same chemicals also
　　help caterpillars survive by disabling
　　the wilt virus. Forest stands of red
(60) oaks, with their tannin-rich foliage,
　　may even provide caterpillars with
　　safe havens from disease. In stands
　　dominated by trees such as aspen,
　　however, incipient gypsy moth outbreaks
(65) are quickly suppressed by viral
　　epidemics.
　　　Further research has shown that cater-
　　pillars become virtually immune to the
　　wilt virus as the trees on which they
(70) feed respond to increasing defoliation.
　　The trees own defenses raise the
　　threshold of caterpillar vulnerability
　　to the disease, allowing populations
　　to grow denser without becoming
(75) more susceptible to infection. For
　　these reasons, the benefits to the
　　caterpillars of ingesting phenols
　　appear to outweigh die costs. Given
　　the presence of the virus, the trees
(80) defensive tactic apparently has
　　backfired.

Which of the following statements best
expresses the main point of the passage?
Recurring outbreaks of infestation by
gypsy moth caterpillars have had a
devastating impact on trees in the
northeastern United States.
A mechanism used by trees to combat
the threat from gypsy moth caterpillars
has actually made some trees more
vulnerable to that threat.
Although deadly to gypsy moth cater-
pillars, wilt disease has failed to
significantly affect the population
density of the caterpillars.
The tree species with the highest levels
of phenols in their foliage are the most
successful in defending themselves
against gypsy moth caterpillars.
In their efforts to develop new,
methods for controlling gypsy moth
caterpillars, researchers have focused
on the effects of phenols in tree
leaves on the insects growt