A small number of the forest
species of lepidoptera (moths and
butterflies, which exist as caterpillars
Line during most of their life cycle) exhibit
(5) regularly recurring patterns of popu-
lation growth and decline—such
fluctuations in population are known
as population cycles. Although many
different variables influence popula-
(10) tion levels, a regular pattern such as
a population cycle seems to imply a
dominant, driving force. Identification
of that driving force, however, has
proved surprisingly elusive despite
(15) considerable research. The com-
mon approach of studying causes of
population cycles by measuring the
mortality caused by different agents,
such as predatory birds or parasites,
(20) has been unproductive in the case of
lepidoptera. Moreover, population
ecologists’ attempts to alter cycles
by changing the caterpillars’ habitat
and by reducing caterpillar popula-
(25) tions have not succeeded. In short,
the evidence implies that these insect
populations, if not self-regulating, may
at least be regulated by an agent more
intimately connected with the insect than
(30) are predatory birds or parasites.
Recent work suggests that this
agent may be a virus. For many
years, viral disease had been
reported in declining populations
(35) of caterpillars, but population ecolo-
gists had usually considered viral
disease to have contributed to the
decline once it was underway rather
than to have initiated it. The recent
(40) work has been made possible by
new techniques of molecular biology
that allow viral DNA to be detected
at low concentrations in the environ-
ment. Nuclear polyhedrosis viruses
(45) are hypothesized to be the driving
force behind population cycles in
lepidoptera in part because the
viruses themselves follow an infec-
tious cycle in which, if protected from
(50) direct sun light, they may remain
virulent for many years in the envi-
ronment, embedded in durable
crystals of polyhedrin protein.
Once ingested by a caterpillar,
(55) the crystals dissolve, releasing
the virus to infect the insect’s cells.
Late in the course of the infection,
millions of new virus particles are
formed and enclosed in polyhedrin
(60) crystals. These crystals reenter the
environment after the insect dies and
decomposes, thus becoming avail-
able to infect other caterpillars.
One of the attractions of this
(65) hypothesis is its broad applicability.
Remarkably, despite significant differ-
ences in habitat and behavior, many
species of lepidoptera have population
cycles of similar length, between eight
(70) and eleven years. Nuclear polyhe-
drosis viral infection is one factor these
disparate species share.
37。According to the passage, before the discovery of new techniques for detecting viral DNA, population ecologists believed that viral diseases
- were not widely prevalent among insect populations generally
- affected only the caterpillar life stage of lepidoptera
- were the driving force behind lepidoptera population cycles
- attacked already declining caterpillar populations
- infected birds and parasites that prey on various species of lepidoptera
查了前人的帖子， 没有怎么讨论37， 只是说答案是D
但是我认为答案是B， 理由：
For many
years, viral disease had been
reported in declining populations
(35) of caterpillars, but population ecolo-
gists had usually considered viral
disease to have contributed to the
decline once it was underway rather
than to have initiated it. | 
