Silent Spring

by Rachel Carson

For the first time in the history of the world, every human being is now subjected to contact with dangerous chemicals, from the moment of conception until death. In the less than two decades of their use, the synthetic pesticides have been so thoroughly distributed throughout the animate and inanimate world that they occur virtually everywhere. They have been recovered from most of the major river systems and even from streams of groundwater flowing unseen through the earth. Residues of these chemicals linger in soil to which they may have been applied a dozen years before. They have entered and lodged in the bodies of fish, birds, reptiles, and domestic and wild animals so universally that scientists carrying on animal experiments find it almost impossible to locate subjects free from such contamination. They have been found in fish in remote mountain lakes, in earthworms burrowing in soil, in the eggs of birds—and in man himself. For these chemicals are now stored in the bodies of the vast majority of human beings, regardless of age. They occur in the mother’s milk, and probably in the tissues of the unborn child. All this has come about because of the sudden rise and prodigious growth of an industry for the production of man-made or synthetic chemicals with insecticidal properties. This industry is a child of the Second World War. In the course of developing agents of chemical warfare, some of the chemicals created in the laboratory were found to be lethal to insects. ...

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What sets the new synthetic insecticides apart is their enormous biological potency. They have immense power not merely to poison but to enter into the most vital processes of the body and change them in sinister and often deadly ways. Thus, as we shall see, they destroy the very enzymes whose function is to protect the body from harm, they block the oxidation processes from which the body receives its energy, they prevent the normal functioning of various organs, and they may initiate in certain cells the slow and irreversible change that leads to malignancy.

Modern insecticides are still more deadly. The vast majority fall into one of two large groups of chemicals. One, represented by DDT, is known as the “chlorinated hydrocarbons.” The other group consists of the organic phosphorus insecticides, and is represented by the reasonably familiar malathion and parathion. All have one thing in common. As mentioned above, they are built on a basis of carbon atoms, which are also the indispensable building blocks of the living world, and thus classed as “organic.” To understand them, we must see of what they are made, and how, although linked with the basic chemistry of all life, they lend themselves to the modifications which make them agents of death. The basic element, carbon, is one whose atoms have an almost infinite capacity for uniting with each other in chains and rings and various other configurations, and for becoming linked with atoms of other substances. Indeed, the incredible diversity of living creatures from bacteria to the great blue whale is largely due to this capacity of carbon. The complex protein molecule has the carbon atom as its basis, as have molecules of fat, carbohydrates, enzymes, and vitamins. So, too, have enormous numbers of nonliving things, for carbon is not necessarily a symbol of life. Some organic compounds are simply combinations of carbon and hydrogen. The simplest of these is methane, or marsh gas, formed in nature by the bacterial decomposition of organic matter under water. Mixed with air in pr...

Substitute chlorine atoms for all of the hydrogen atoms and the result is carbon tetrachloride, the familiar cleaning fluid: In the simplest possible terms, these changes rung upon the basic molecule of methane illustrate what a chlorinated hydrocarbon is. But this illustration gives little hint of the true complexity of the chemical world of the hydrocarbons, or of the manipulations by which the organic chemist creates his infinitely varied materials. For instead of the simple methane molecule with its single carbon atom, he may work with hydrocarbon molecules consisting of many carbon atoms, arranged in rings or chains, with side chains or branches, holding to themselves with chemical bonds not merely simple atoms of hydrogen or chlorine but also a wide variety of chemical groups.

As long ago as the mid-1930’s a special group of hydrocarbons, the chlorinated naphthalenes, was found to cause hepatitis, and also a rare and almost invariably fatal liver disease in persons subjected to occupational exposure. They have led to illness and death of workers in electrical industries; and more recently, in agriculture, they have been considered a cause of a mysterious and usually fatal disease of cattle. In view of these antecedents, it is not surprising that three of the insecticides that are related to this group are among the most violently poisonous of all the hydrocarbons. These are dieldrin, aldrin, and endrin. Dieldrin, named for a German chemist, Diels, is about 5 times as toxic as DDT when swallowed but 40 times as toxic when absorbed through the skin in solution. It is notorious for striking quickly and with terrible effect at the nervous system, sending the victims into convulsions. Persons thus poisoned recover so slowly as to indicate chronic effects. As with other chlorinated hydrocarbons, these long-term effects include severe damage to the liver. The long duration of its residues and the effective insecticidal action make dieldrin one of the most used insecticides today, despite the appalling destruction of wildlife that has followed its use. As tested on quail and pheasants, it has proved to be about 40 to 50 times as toxic as DDT. There are vast gaps in our knowledge of how dieldrin is stored or distributed in the body, or excreted, for the ...

Aldrin is a somewhat mysterious substance, for although it exists as a separate entity it bears the relation of alter ego to dieldrin. When carrots are taken from a bed treated with aldrin they are found to contain residues of dieldrin. This change occurs in living tissues and also in soil. Such alchemistic transformations have led to many erroneous reports, for if a chemist, knowing aldrin has been applied, tests for it he will be deceived into thinking all residues have been dissipated. The residues are there, but they are dieldrin and this requires a different test.

The question of chemical residues on the food we eat is a hotly debated issue. The existence of such residues is either played down by the industry as unimportant or is flatly denied. Simultaneously, there is a strong tendency to brand as fanatics or cultists all who are so perverse as to demand that their food be free of insect poisons. In all this cloud of controversy, what are the actual facts?

They are heaviest in butter and other manufactured dairy products.

The fat samples were taken from people who had left their native villages to enter the United States Public Health Service Hospital in Anchorage for surgery. There the ways of civilization prevailed, and the meals in this hospital were found to contain as much DDT as those in the most populous city. For their brief stay in civilization the Eskimos were rewarded with a taint of poison.

a lettuce farmer who applied not one but eight different insecticides to his crop within a short time of harvest, a shipper who had used the deadly parathion on celery in an amount five times the recommended maximum, growers using endrin—most toxic of all the chlorinated hydrocarbons—on lettuce although no residue was allowable, spinach sprayed with DDT a week before harvest.

The trouble is that we are seldom aware of the protection afforded by natural enemies until it fails. Most of us walk unseeing through the world, unaware alike of its beauties, its wonders, and the strange and sometimes terrible intensity of the lives that are being lived about us. So it is that the activities of the insect predators and parasites are known to few. Perhaps we may have noticed an oddly shaped insect of ferocious mien on a bush in the garden and been dimly aware that the praying mantis lives at the expense of other insects.

Then in the 1940’s the citrus growers began to experiment with glamorous new chemicals against other insects. With the advent of DDT and the even more toxic chemicals to follow, the populations of the vedalia in many sections of California were wiped out. Its importation had cost the government a mere $5000. Its activities had saved the fruit growers several millions of dollars a year, but in a moment of heedlessness the benefit was canceled out. Infestations of the scale insect quickly reappeared and damage exceeded anything that had been seen for fifty years.

It is well on the way to realizing the philosophy stated by the Canadian entomologist G. C. Ullyett a decade ago: “We must change our philosophy, abandon our attitude of human superiority and admit that in many cases in natural environments we find ways and means of limiting populations of organisms in a more economical way than we can do it ourselves.”

Although insect resistance is a matter of concern in agriculture and forestry, it is in the field of public health that the most serious apprehensions have been felt. The relation between various insects and many diseases of man is an ancient one. Mosquitoes of the genus Anopheles may inject into the human bloodstream the single-celled organism of malaria. Other mosquitoes transmit yellow fever. Still others carry encephalitis. The housefly, which does not bite, nevertheless by contact may contaminate human food with the bacillus of dysentery, and in many parts of the world may play an important part in the transmission of eye diseases. The list of diseases and their insect carriers, or vectors, includes typhus and body lice, plague and rat fleas, African sleeping sickness and tsetse flies, various fevers and ticks, and innumerable others.

The question that has now urgently presented itself is whether it is either wise or responsible to attack the problem by methods that are rapidly making it worse.

Probably the first medical use of modern insecticides occurred in Italy in 1943 when the Allied Military Government launched a successful attack on typhus by dusting enormous numbers of people with DDT. This was followed two years later by extensive application of residual sprays for the control of malaria mosquitoes. Only a year later the first signs of trouble appeared. Both houseflies and mosquitoes of the genus Culex began to show resistance to the sprays. In 1948 a new chemical, chlordane, was tried as a supplement to DDT. This time good control was obtained for two years, but by August of 1950 chlordane-resistant flies appeared, and by the end of that year all of the houseflies as well as the Culex mosquitoes seemed to be resistant to chlordane.

By the end of 1951, DDT, methoxychlor, chlordane, heptachlor, and benzene hexachloride had joined the list of chemicals no longer effective. The flies, meanwhile, had become “fantastically abundant.”

The first malaria mosquito to develop resistance to DDT was Anopheles sacharovi in Greece. Extensive

The ordinary house mosquito is here and there developing resistance, a fact that should give pause to many communities that now regularly arrange for wholesale spraying. This species is now resistant to several insecticides, among which is the almost universally used DDT, in Italy, Israel, Japan, France, and parts of the United States, including California, Ohio, New Jersey, and Massachusetts.

Darwin himself could scarcely have found a better example of the operation of natural selection than is provided by the way the mechanism of resistance operates. Out of an original population, the members of which vary greatly in qualities of structure, behavior, or physiology, it is the “tough” insects that survive chemical attack. Spraying kills off the weaklings. The only survivors are insects that have some inherent quality that allows them to escape harm.

The means by which insects resist chemicals probably vary and as yet are not thoroughly understood. Some of the insects that defy chemical control are thought to be aided by a structural advantage, but there seems to be little actual proof of this. That immunity exists in some strains is clear, however, from observations like those of Dr. Briejèr, who reports watching flies at the Pest Control Institute at Springforbi, Denmark, “disporting themselves in DDT as much at home as primitive sorcerers cavorting over red-hot coals.”

Nevertheless, the quality of resistance does not necessarily depend on physical structure. DDT-resistant flies possess an enzyme that allows them to detoxify the insecticide to the less toxic chemical DDE. This enzyme occurs only in flies that possess a genetic factor for DDT resistance. This factor is, of course, hereditary. How flies and other insects detoxify the organic phosphorus chemicals is less clearly understood.

Ordinarily resistance takes two or three years to develop, although occasionally it will do so in only one season, or even less. At the other extreme it may take as long as six years. The number of generations produced by an insect population in a year is important, and this varies with species and climate. Flies in Canada, for example, have been slower to develop resistance than those in southern United States, where long hot summers favor a rapid rate of reproduction.

Theoretically they could; but since this would take hundreds or even thousands of years, the comfort to those living now is slight. Resistance is not something that develops in an individual. If he possesses at birth some qualities that make him less susceptible than others to poisons he is more likely to survive and produce children. Resistance, therefore, is something that develops in a population after time measured in several or many generations. Human populations reproduce at the rate of roughly three generations per century, but new insect generations arise in a matter of days or weeks.

Plant Protection Service.

The Department of Agriculture’s Yearbook for 1952, devoted entirely to insects, recognizes the fact that insects become resistant but says, “More applications or greater quantities of the insecticides are needed then for adequate control.” The Department does not say what will happen when the only chemicals left untried are those that render the earth not only insectless but lifeless. But in 1959, only seven years after this advice was given, a Connecticut entomologist was quoted in the Journal of Agricultural and Food Chemistry to the effect that on at least one or two insect pests the last available new material was then being used. Dr. Briejèr says: It is more than clear that we are traveling a dangerous road. . . . We are going to have to do some very energetic research on other control measures, measures that will have to be biological, not chemical. Our aim should be to guide natural processes as cautiously as possible in the desired direction rather than to use brute force. . . . We need a more high-minded orientation and a deeper insight, which I miss in many researchers. Life is a miracle beyond our comprehension, and we should reverence it even where we have to struggle against it. . . . The resort to weapons such as insecticides to control it is a proof of insufficient knowledge and of an incapacity so to guide the processes of nature that brute force becomes unnecessary. Humbleness is in order; there is no excuse for scientific conceit here.

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.

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 Knipling, and his associates.

Beginning in August 1954, screw-worms reared and sterilized in an Agriculture Department laboratory in Florida were flown to Curaçao and released from airplanes at the rate of about 400 per square mile per week. Almost at once the number of egg masses deposited on experimental goats began to decrease, as did their fertility. Only seven weeks after the releases were started, all eggs were infertile. Soon it was impossible to find a single egg mass, sterile or otherwise. The screw-worm had indeed been eradicated on Curaçao.

triumphant demonstration of the worth of scientific creativity, aided by thorough basic research, persistence, and determination.

One of the problems of sterilization by radiation is that this requires not only artificial rearing but the release of sterile males in larger number than are present in the wild population. This could be done with the screw-worm, which is actually not an abundant insect. With the housefly, however, more than doubling the population through releases could be highly objectionable, even though the increase would be only temporary.

The sterilants currently being tested fall generally into two groups, both of which are extremely interesting in their mode of action. The first are intimately related to the life processes, or metabolism, of the cell; i.e., they so closely resemble a substance the cell or tissue needs that the organism “mistakes” them for the true metabolite and tries to incorporate them in its normal building processes. But the fit is wrong in some detail and the process comes to a halt. Such chemicals are called antimetabolites. The second group consists of chemicals that act on the chromosomes, probably affecting the gene chemicals and causing the chromosomes to break up. The chemosterilants of this group are alkylating agents, which are extremely reactive chemicals, capable of intense cell destruction, damage to chromosomes, and production of mutations. It is the view of Dr. Peter Alexander of the Chester Beatty Research Institute in London that “any alkylating agent which is effective in sterilizing insects would also be a powerful mutagen and carcinogen.” Dr. Alexander feels that any conceivable use of such chemicals in insect control would be “open to the most severe objections.” It is to be hoped, therefore, that the present experiments will lead not to actual use of these particular chemicals but to the discovery of others that will be safe and also highly specific in their action on the target insect.

Dr. Edward Steinhaus, an outstanding authority on insect pathology, has stated emphatically that there is “no authenticated recorded instance of a true insect pathogen having caused an infectious disease in a vertebrate animal either experimentally or in nature.”

In the United States the true beginnings of conventional biological control date from 1888 when Albert Koebele, the first of a growing army of entomologist explorers, went to Australia to search for natural enemies of the cottony cushion scale that threatened the California citrus industry with destruction.

Yet biological control has suffered from lack of support. California is virtually alone among the states in having a formal program in biological control, and many states have not even one entomologist who devotes full time to it.

The Canadians take a broader view, and some of the Europeans have gone farthest of all to develop the science of “forest hygiene” to an amazing extent. Birds, ants, forest spiders, and soil bacteria are as much a part of a forest as the trees, in the view of European foresters, who take care to inoculate a new forest with these protective factors. The encouragement of birds is one of the first steps. In the modern era of intensive forestry the old hollow trees are gone and with them homes for woodpeckers and other tree-nesting birds. This lack is met by nesting boxes, which draw the birds back into the forest. Other boxes are specially designed for owls and for bats, so that these creatures may take over in the dark hours the work of insect hunting performed in daylight by the small birds.

“Where you can obtain in your forest a combination of birds’ and ants’ protection together with some bats and owls, the biological equilibrium has already been essentially improved,” says Dr. Heinz Ruppertshofen, a forestry officer in Mölln, Germany, who believes that a single introduced predator or parasite is less effective than an array of the “natural companions” of the trees.

Much of the work of caring for the ant colonies (and the birds’ nesting boxes as well) is assumed by a youth corps from the local school, children 10 to 14 years old. The costs are exceedingly low;

There is, then, a whole battery of armaments available to the forester who is willing to look for permanent solutions that preserve and strengthen the natural relations in the forest.

Through all these new, imaginative, and creative approaches to the problem of sharing our earth with other creatures there runs a constant theme, the awareness that we are dealing with life—with living populations and all their pressures and counterpressures, their surges and recessions. Only by taking account of such life forces and by cautiously seeking to guide them into channels favorable to ourselves can we hope to achieve a reasonable accommodation between the insect hordes and ourselves.

The Ecology of Invasions by Animals and Plants.

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“Insecticide Storage in Adipose Tissue,” editorial, Jour. Am. Med. Assn., Vol. 145 (March 10, 1951), pp. 735–36.

Mitchell, Philip H., A Textbook of General Physiology. New York: McGraw-Hill, 1956. 5th ed. 191 Miller, B. F., and R. Goode, Man and His Body: The Wonders of the Human Mechanism. New York: Simon and Schuster, 1960.

“Animal Mitochondria,” Annual Rev. Physiol., Vol. 20

Green, David EL, “Biological Oxidation,” Sci. American, Vol. 199 (1958), No. 1, pp. 56–62.

Elton, Charles S., The Ecology of Invasions by Animals and Plants. New York: Wiley, 1958. P. 181.

Briggs, John D., “Pathogens for the Control of Pests,” Biol, and Chem. Control of Plant and Animal Pests. Washington, D.C., Am. Assn. Advancement Sci., 1960. Pp. 137–48.