2021年10月 / 马哥(Marlin)整理
- The Other Road / 另一条道路
(部分节选 - 可能不完整)
We stand now where two roads diverge. But unlike the roads in Robert Frost's familiar poem, they are not equally fair. The road we have long been traveling is deceptively easy, a smooth superhighway on which we progress with great speed, but at its end lies disaster. The other fork of the road — the one "less traveled by" — offers our last, our only chance to reach a destination that assures the preservation of our earth.
The choice, after all, is ours to make. If, having endured much, we have at last asserted our "right to know", and if, knowing, we have concluded that we are being asked to take senseless and frightening risks, then we should no longer accept the counsel of those who tell us that we must fill our world with poisonous chemicals; we should look about and see what other course is open to us.
A truly extraordinary variety of alternatives to the chemical control of insects is available. Some are already in use and have achieved brilliant success. Others are in the stage of laboratory testing. Still others are little more than ideas in the minds of imaginative scientists, waiting for the opportunity to put them to the test. All have this in common: they are biological solutions, based on understanding of the living organisms they seek to control, and of the whole fabric of life to which these organisms belong. Specialists representing various areas of the vast field of biology are contributing — entomologists, pathologists, geneticists, physiologists, biochemists, ecologists — all pouring their knowledge and their creative inspirations into the formation of a new science of biotic controls.
"Any science may be likened to a river," says a Johns Hopkins biologist, Professor Carl P. Swanson. "It has its obscure and unpretentious beginning; its quiet stretches as well as its rapids; its periods of drought as well as of fullness. It gathers momentum with the work of many investigators and as it is fed by other streams of thought; it is deepened and broadened by the concepts and generalizations that are gradually evolved."
This is particularly true of the science of biotic controls in its modern sense. In America it had its small beginnings a century ago with the first attempts to introduce natural enemies of insects that were proving troublesome to farmers, an effort that sometimes moved slowly or not at all, but now and again gathered speed and momentum under the impetus of an outstanding success. It had its period of drought when workers in applied entomology, dazzled by the spectacular new insecticides of the 1940's, turned their backs on all biological methods and set foot on "the treadmill of chemical control." But the goal of an insect-free world continued to recede. Now at last, as it has become apparent that the heedless and unrestrained use of chemicals is a greater menace to ourselves than to the targets, the river which is the science of biotic controls is again flowing full and free.
Some of the most fascinating of the new methods are those that seek to turn the strength of a species against itself — to use the drive of an insect's life forces to destroy it. The most spectacular of these approaches is the "male sterilization" technique developed by the chief of the United States Department of Agriculture's Entomology Research Branch, Dr. Edward F. Knipling, and his associates.
About a quarter of a century ago Dr. Knipling startled his colleagues by proposing a unique method of insect control. If it were possible to sterilize and release large numbers of male insects, he theorized, the sterilized males would, under certain conditions, compete with the normal wild males so successfully that, after repeated releases, only infertile eggs would be produced and the population would die out.
The proposal was met with bureaucratic inertia and with skepticism from scientists, but the idea persisted in Dr. Knipling's mind. One major problem remained to be solved before it could be put to the test — a practical method of insect sterilization had to be found. Academically, the fact that insects could be sterilized by exposure to X-ray had been known since 1916, when an entomologist by the name of G. A. Runner reported such sterilization of cigarette beetles. Hermann Muller's pioneering work on the production of mutations by X-ray opened up vast new areas of thought in the late 1920's, and by the middle of the century various workers had reported the sterilization by X-rays or gamma rays of at least a dozen species of insects.
But these were laboratory experiments, still a long way from practical application. About 1950, Dr. Knipling launched a serious effort to turn insect sterilization into a weapon that would wipe out a major insect pest. The Department of Agriculture provided some funds, and an intensive research program was begun at the Orlando, Florida, laboratory of the Department's Entomology Research Branch, with the help of Dr. Raymond C. Bushland, one of Dr. Knipling's associates. Dr. Bushland and Dr. Knipling were well acquainted with the reproductive habits of the screwworm fly, which is born in the living tissues of warm-blooded animals, primarily cattle, and causes extremely serious damage to livestock in the southeastern part of the United States and in Mexico. It was to control this insect pest that they first directed their energies.
The Screwworm fly, Cochliomyia hominivorax, is a handsome gaily colored greenish-blue fly, much like the common housefly in size and appearance. Its name is derived from the fact that the larva is armed with small hooks or spines on the segments of its body, by means of which it attaches itself to the flesh of its host. The mature female deposits her eggs on the edges of wounds or scratches in the skin of cattle, and in these sores the larvae hatch and feed. A heavy infestation may mean the death of the animal, and those that survive an attack are so weakened by loss of blood and damaged tissue that they become more susceptible to other diseases. The annual loss to the livestock industry in the United States from screwworm infestation amounts to about $40,000,000.
The early work on screwworm sterilization at the Orlando laboratory was encouraging. Dr. Bushland found that by exposing the flies to gamma rays from a radioactive isotope of cobalt it was possible to sterilize them without causing any adverse effects on their ability to mate. The sterilized males could then be released in a given area to compete with the normal wild males in mating with the females. The project advanced to the stage where it was ready for a field test. For this, some three and a half square miles of the island of Curaçao, off the coast of Venezuela, were selected in 1954 as the test area. The choice of Curaçao had several advantages: although the screwworm is native to the southwestern United States, it had been accidentally introduced into Curaçao and had become a pest there; furthermore, the island was small enough to be made reasonably free of screwworm flies by a combination of insecticide treatment and the release of sterile males.
The project was carried out with every precaution to make it a fair test. The native screwworm flies were first eradicated by the release of sterilized males in an area of about 160 square miles, including both the experimental area and the areas immediately surrounding it. Then, in the spring of 1954, after the last native fly had disappeared, sterile males were released at the rate of about 400 per square mile per week in the experimental area. The native population of flies soon began to reappear in the surrounding areas, and these flies then migrated into the experimental area, where they mated with the sterile males. As a result, throughout the season the percentage of eggs that hatched was extremely low, and the next spring there were so few flies that the release of sterile males was discontinued. The project had been a complete success. The few screwworm flies that were found the following year were sterile hybrids.
The Screwworm fly was the first insect to be eradicated from a region by the sterile male technique, but soon others were to follow. The list is growing steadily and includes some of the world's most important insect pests. For example, the melon fly, which infests various fruits in Hawaii and the southwestern Pacific, has been successfully suppressed by this method. The oriental fruit fly, a serious pest in many areas of the world, is being held in check by sterile male releases in the Mariana Islands, in the Bonin Islands, and in parts of the Philippines. Sterile males have also been released in Panama and in Guatemala to combat the Mediterranean fruit fly, which attacks a wide variety of fruits and vegetables, and there are serious plans to use this method against the citrus tristeza virus in Mexico and Central America.
The sterile male technique has been tried on several species of mosquitoes, and some of the most promising results have been obtained with the tsetse fly in Africa. In Uganda, the tsetse fly transmits the trypanosome parasite that causes sleeping sickness in man and a related disease in domestic animals. The use of chemicals to control the tsetse fly has been only partly successful, and the search for a better method has been urgent. The release of sterile male tsetse flies is now being tested in the Tororo Reserve in Uganda.
The application of the sterile male technique against the common housefly has also been proposed. The housefly, Musca domestica, is a cosmopolitan insect that has become adapted to living in close association with man and is a carrier of many diseases, including typhoid fever, dysentery, cholera, and anthrax. Although the housefly is not as serious a pest as some of the insects mentioned above, it is nevertheless a nuisance and a potential health hazard. The sterile male technique could be used to reduce the housefly population in areas where other methods of control have not been successful.
The sterile male technique is not the only method of biotic control that shows promise. Another approach is the use of hormones or other chemicals that interfere with the normal development or behavior of insects. For example, there are chemicals that can prevent insects from molting or pupating, or that can disrupt their mating behavior. These chemicals, which are known as insect growth regulators or behavior-modifying chemicals, are highly specific in their action and have little or no effect on other organisms. They offer the possibility of controlling insect pests without harming beneficial insects or other wildlife.
One of the most interesting of these chemicals is a substance called juvenile hormone. Juvenile hormone is secreted by certain glands in the insect's body and controls its growth and development. If an insect is exposed to an excess of juvenile hormone, it will continue to grow and molt but will not pupate or become an adult. Instead, it will remain in a juvenile or larval stage until it eventually dies. This phenomenon has been exploited by scientists to develop methods of insect control. For example, a synthetic juvenile hormone has been developed that can be sprayed on plants to prevent the larvae of certain insects from developing into adults. This method has been tested successfully against the tobacco hornworm, a serious pest of tobacco plants, and shows promise for the control of other insect pests as well.
Another approach to biotic control is the use of natural enemies of insects, such as predators, parasites, and pathogens. This method has been used for centuries by farmers and gardeners, who have observed that certain insects prey on or parasitize other insects and have used this knowledge to control pest populations. For example, ladybugs are well known for their appetite for aphids, and many gardeners release ladybugs in their gardens to control aphid infestations. Similarly, parasitic wasps are used to control the larvae of many insects, including the gypsy moth and the codling moth.
In recent years, there has been a growing interest in the use of pathogens, such as bacteria, viruses, and fungi, to control insect pests. These pathogens are highly specific in their action and can cause diseases that are fatal to certain insects but have no effect on other organisms. For example, Bacillus thuringiensis, a bacterium that is widely used to control the larvae of moths and butterflies, produces a toxin that is lethal to these insects but harmless to other animals, including man. Similarly, a virus that attacks the gypsy moth has been used successfully to control this pest in some areas.
The use of natural enemies of insects has several advantages over chemical control methods. First, natural enemies are highly specific in their action and do not harm beneficial insects or other wildlife. Second, natural enemies can provide long-term control of insect pests, since they can reproduce and maintain their populations in the environment. Third, the use of natural enemies is less likely to lead to the development of resistance in insect pests, since the pests are being attacked by a complex of natural enemies rather than by a single chemical.
However, the use of natural enemies also has some limitations. First, it may be difficult to find and identify the natural enemies of a particular insect pest, especially if the pest is an exotic species that has been introduced into a new area. Second, it may be difficult to mass-produce natural enemies in sufficient quantities to control large pest populations. Third, the release of natural enemies may be affected by environmental factors such as weather and habitat, which can limit their effectiveness.
Despite these limitations, the use of natural enemies of insects is an important component of biotic control and offers many advantages over chemical control methods. In many cases, a combination of different biotic control methods may be the most effective way to control insect pests. For example, the release of sterile males may be combined with the use of natural enemies or with the application of insect growth regulators to achieve better control of pest populations.
The science of biotic controls is still in its infancy, but it holds great promise for the future. As our understanding of the biology of insects and their interactions with other organisms increases, we can expect to see the development of even more effective and environmentally friendly methods of insect control. The choice is ours to make. We can continue on the road we have been traveling, which leads to disaster, or we can choose the other road — the road of biotic controls — which offers hope for the preservation of our earth and all its living things.