| Line | A small number of the forest species of |
| | lepidoptera (moths and butterflies, which exist as |
| | caterpillars during most of their life cycle) exhibit |
| | regularly recurring patterns of population growth |
| (5) | and decline—such fluctuations in population are |
| | known as population cycles. Although many different |
| | variables influence population levels, a regular pattern |
| | such as a population cycle seems to imply a |
| | dominant, driving force. Identification of that driving |
| (10) | force, however, has proved surprisingly elusive |
| | despite considerable research. The common |
| | approach of studying causes of population cycles by |
| | measuring the mortality caused by different agents, |
| | such as predatory birds or parasites, has been |
| (15) | unproductive in the case of lepidoptera. Moreover, |
| | population ecologists attempts to alter cycles by |
| | changing the caterpillars habitat and by reducing |
| | caterpillar populations have not succeeded. In short, |
| | the evidence implies that these insect populations, if |
| (20) | not self-regulating, may at least be regulated by an |
| | agent more intimately connected with the insect than |
| | are predatory birds or parasites. |
| | Recent work suggests that this agent may be a |
| | virus. For many years, viral disease had been reported |
| (25) | in declining populations of caterpillars, but population |
| | ecologists had usually considered viral disease to |
| | have contributed to the decline once it was underway |
| | rather than to have initiated it. The recent work has |
| | been made possible by new techniques of molecular |
| (30) | biology that allow viral DNA to be detected at low |
| | concentrations in the environment. Nuclear |
| | polyhedrosis viruses are hypothesized to be the |
| | driving force behind population cycles in lepidoptera |
| | in part because the viruses themselves follow an |
| (35) | infectious cycle in which, if protected from direct |
| | sun light, they may remain virulent for many years |
| | in the environment, embedded in durable crystals of |
| | polyhedrin protein. Once ingested by a caterpillar, |
| | the crystals dissolve, releasing the virus to infect |
| (40) | the insects cells. Late in the course of the infection, |
| | millions of new virus particles are formed and |
| | enclosed in polyhedrin crystals. These crystals |
| | reenter the environment after the insect dies and |
| | decomposes, thus becoming available to infect |
| (45) | other caterpillars. |
| | One of the attractions of this hypothesis is its broad |
| | applicability. Remarkably, despite significant differences |
| | in habitat and behavior, many species of lepidoptera |
| | have population cycles of similar length, between eight |
| (50) | and eleven years. Nuclear polyhedrosis viral infection is |
| | one factor these disparate species share. |