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Unformatted text preview: e infestations of gypsy moth caterpillars is a recurring virtually immune to the wilt virus as the trees on which phenomenon in the northeastern United States. In studying they feed respond to increasing defoliation. The trees' own these outbreaks, scientists have discovered that affected defenses raise the threshold of caterpillar vulnerability to trees fight back by releasing toxic chemicals, mainly the disease, allowing populations to grow denser without phenols, into their foliage. These noxious substances limit becoming more susceptible to infection. For these reasons, caterpillars' growth and reduce the number of eggs that the benefits to the caterpillars of ingesting phenols appear female moths lay. Phenols also make the eggs s maller, to outweigh the costs. Given the presence of the virus, the which reduces the growth of the following year's trees' d efensive tactic apparently has backfired. caterpillars. Because the number of eggs a female moth (446 words) produces is directly related to her size, and because her
size is determined entirely by her feeding success as a
caterpillar, the trees' defensive mechanism has an impact
on moth fecundity.
The gypsy moth is also subject to attack by the
nucleopolyhedrosis virus, or wilt disease, a particularly
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 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 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.
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 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. 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 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
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 are quickly suppressed by viral epidemics.
176 2. It can be inferred from the passage that wilt disease 4. Select the sentence in the passage that the author uses virions depend for their survival on as a supporting idea to explicate how gypsy...
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