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 thesize 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 |
购买
请下载安装
网友评论：（只显示最新5条。评论内容只代表网友观点，与本站立场无关！）