In other words, one one-hundredth of the lethal dose of each compound can be fatal when the two are combined. Potentiation seems to take place when one compound destroys the liver enzyme responsible for detoxicating the other. The two need not be given simultaneously. And the hazard exists not only for the man who may spray this week with one insecticide and next week with another; it exists also for the consumer of sprayed products.
The common salad bowl may easily present a combination of organic-phosphate insecticides in quantities large enough to interact. In Greek mythology, the sorceress Medea, enraged at being supplanted by a rival in the affections of her husband, Jason, presented the new bride with a robe possessing magical properties The wearer of the robe immediately suffered a violent death. The purpose is to kill insects that may come in contact with these poisonous beings, especially by sucking their juices or their blood.
The world of systemic insecticides is a weird world, surpassing the imaginings of the brothers Grimm. It is a world where the enchanted forest of the fairy tales has become a poisonous forest. It is a world where a flea bites a dog and dies, where an insect may die as a result of chewing a leaf or inhaling vapors emanating from a plant it has never touched, where a bee may carry poisonous nectar back to its hive and presently produce poisonous honey.
Selenium, a naturally occurring element found sparingly in rocks and soils of many parts of the world, thus became the first systemic insecticide. What makes an insecticide a systemic is its ability to permeate all the tissues of a plant or animal and make them toxic.
This quality is possessed by some chemicals of the chlorinated-hydrocarbon group and by others of the organic-phosphate group, all synthetically produced. In practice, most systemics are drawn from the organic-phosphate group, because with these the problem of residues is somewhat less acute.
Systemics can act in devious ways. Applied to seeds, either by soaking or by means of a coating in which the systemic is combined with carbon, they extend their effects into the following plant generation and produce seedlings poisonous to aphids and other sucking insects. Such vegetables as peas, beans, and sugar beets are sometimes thus protected. Cotton seeds coated with a systemic called phorate have been in use for some time in California, and in twenty-five farm laborers in the San Joaquin Valley, who had handled bags of treated seeds, were seized with sudden illness.
In England, someone wondered what happened when bees made use of nectar from plants that had been treated with systemics. This was investigated in areas treated with a chemical called schradan.
Although the plants had been sprayed before the flowers were formed, the nectar they produced contained the poison. The result, as might have been predicted, was that the honey made by the bees was also contaminated with schradan. Animal systemics have been used chiefly to control the cattle grub, a damaging parasite of livestock. Extreme care must be taken in order to create an insecticidal effect in the blood and tissues of the host without setting up a fatal poisoning.
As yet, no one seems to have proposed a human systemic that would make us lethal to a mosquito. Perhaps this is the next step. When we turn our attention to herbicides, or weed killers, we quickly come across the legend that they are toxic only to plants. Unfortunately, this is only a legend. The plant killers include a large variety of chemicals that act on animal tissue as well as on vegetation. No general statement can describe the action of all of them.
Some are general poisons; some are powerful stimulants of metabolism, causing a fatal rise in body temperature; some can induce malignant tumors, either alone or in partnership with other chemicals; some can cause gene mutations.
Arsenic compounds are still liberally used, both as insecticides and as weed killers, where they usually take the chemical form of sodium arsenite.
The history of their use is not reassuring. As roadside sprays, they have cost many a farmer his cow and killed uncounted numbers of wild creatures. As aquatic weed killers, they have made public waters unsuitable for drinking, or even for swimming.
As a spray applied to potato fields to destroy the vines, they have taken a toll of human and non-human life. In England, this last practice developed in about , as a result of a shortage of sulphuric acid, which had formerly been used to burn off the potato vines.
The Ministry of Agriculture considered it necessary to issue a warning of the hazard of going into arsenic-sprayed fields, but the warning was not understood by the cattle or by the wild animals and birds , and reports of poisoned cattle were received with monotonous regularity. In , the Australian government announced a similar ban. No such restrictions impede the use of these poisons in the United States. The most widely used herbicides are 2,4-D, 2,4,5-T, and related members of what is known as the phenol group.
Many experts deny that these are toxic. However, people spraying their lawns with 2,4-D and becoming wet with spray have occasionally developed severe neuritis and even paralysis.
Although such incidents are apparently uncommon, medical authorities advise caution in the use of these compounds. Other hazards, more obscure, may also attend the use of 2,4-D. Experiments have shown its ability to disturb the basic physiological process of respiration in the cell, and, like X-rays, to damage the chromosomes.
Some very recent work indicates that sub-lethal doses of these herbicides may affect reproduction in birds. The rest of the phenols may be equally dangerous. Dinitrophenol, for example, steps up the metabolism. For this reason, it was at one time used in the United States as a reducing drug, but the margin between the slimming dose and the dose required to poison or kill was slight—so slight that at least nine patients died and many suffered permanent injury before use of the drug was finally halted.
The fearful power of penta, which acts in much the same way as dinitrophenol, is illustrated in a fatal accident recently reported by the California State Department of Public Health. A man was preparing a cotton defoliant by mixing diesel oil with penta.
As he was drawing the concentrated chemical out of a drum, the spigot accidentally toppled back. He reached in with his bare hand to regain the spigot.
Although he washed immediately, he became acutely ill, and died the next day. Curious indirect results follow the use of certain herbicides. It has been found that animals—both wild herbivores and livestock—are sometimes strangely attracted to a plant that has been sprayed, even though it is not one of their natural foods. Apparently, the wilting that follows spraying or cutting makes the plant attractive. If a highly poisonous herbicide, such as arsenic, has been used, this intense desire to reach the wilting vegetation inevitably has disastrous consequences.
Such consequences may also stem from the use of less toxic herbicides in cases where the plant itself happens to be poisonous or, perhaps, to possess thorns or burs. Poisonous range weeds, for example, have suddenly become attractive to livestock after spraying, and the animals have died from indulging this unnatural appetite.
The literature of veterinary medicine abounds in similar examples: swine eating sprayed cockleburs with consequent severe illness, lambs eating sprayed thistles, bees poisoned by pasturing on mustard that had been sprayed after it came into bloom.
Wild cherry, the leaves of which are highly poisonous, has had a fatal attraction for cattle once its foliage has been sprayed with 2,4-D. The explanation of this peculiar behavior sometimes appears to lie in the changes that the chemical brings about in the metabolism of the plant.
There is a temporary but marked increase in sugar content, and many animals seek the plant out for its sweetness. Another curious effect of 2,4-D has important consequences for livestock and wildlife, and apparently for men as well.
Some of these are normally ignored by cattle but are eaten with relish after treatment with 2,4-D. According to some agricultural specialists, a number of deaths among cattle have been traced to such sprayed weeds.
All ruminants—not only cattle but wild ruminants, such as deer, antelope, sheep, and goats—have a digestive system of extraordinary complexity, including a stomach divided into several chambers. The digestion of cellulose is accomplished in one of the chambers, through the action of microorganisms known as rumen bacteria.
When the animal feeds on vegetation containing nitrates, the rumen bacteria change them into nitrites, and if the level of nitrates is abnormally high, a fatal series of events ensues. When the nitrites are present in large quantities, they act on the blood pigment to form a chocolate-brown substance in which oxygen is so firmly held that it cannot be transferred from the lungs to the tissues. And death occurs within a few hours from anoxia, or lack of oxygen.
In a series of cases studied by the University of Minnesota Medical School, all but one terminated fatally. Of these, water has become the most precious. Ever since chemists began to manufacture substances that nature never invented, the problems of water purification have grown more complex and the danger to users of water has increased. Because they become inextricably mixed with domestic and other wastes, they sometimes defy detection by the standard methods used in purification plants.
Often they cannot be identified, and even if they are, most of them are so stable that they cannot be broken down by ordinary processes. Some are deliberately applied to bodies of water to destroy plants, insect larvae, or undesired fish. Some come from forest spraying, in the course of which two or three million acres of one of our states may be blanketed with spray directed against a single insect pest—spray that falls directly into streams or else drips down through the leafy canopy to the forest floor, there to become part of the slow movement of seeping moisture beginning its long journey to the sea.
Probably the bulk of such contaminants, however, consists of water-borne residues of the millions of pounds of agricultural chemicals that have been leached out of the ground by rains to become part of the same seaward movement. Here and there we have dramatic evidence of the presence of these chemicals in our streams, and even in public water supplies.
A sample of drinking water from an orchard area in Pennsylvania was tested on fish in a laboratory; it contained enough insecticide to kill all the fish within four hours, The runoff from fields treated with a chlorinated hydrocarbon called toxaphene killed all the fish in fifteen streams tributary to the Tennessee River, in Alabama, two of which were sources of municipal water supplies; the water remained poisonous for a week after the application of the insecticide—a fact that was determined by the daily deaths of goldfish suspended in cages downstream.
For the most part, such pollution is invisible; it may make its presence known when hundreds or thousands of fish die, but more often it is never detected at all.
Anyone who doubts that our waters have become almost universally contaminated with insecticides might well study a brief report issued by the United States Fish and Wildlife Service in The Service had carried out studies to discover whether fish, like warm-blooded animals, store insecticides in their tissues.
The first samples were taken from a creek in a forest area in the West where there had been mass spraying of DDT for the control of the spruce budworm. As might have been expected, all these fish contained DDT. The really significant findings were made when the investigators turned for comparison to a remote creek thirty miles from the nearest area sprayed for budworm control.
This creek was upstream from the first, and separated from it by a high waterfall. No local spraying was known to have occurred. Yet the fish in that creek, too, contained DDT. Had the chemical been airborne, drifting down as fallout on the surface of the creek?
Or had it reached the creek by hidden underground streams? Probably no aspect of the entire water-pollution problem is more disturbing than the threat of widespread contamination of ground water. As rain falls on the land, it seeps down through pores and cracks in soil and rock, penetrating deeper and deeper, until eventually it reaches a zone where all the pores of the bedrock are filled with water—a dark, subsurface sea, rising under hills, sinking beneath valleys.
This ground water is always on the move, sometimes as slowly as fifty feet a year, sometimes as rapidly as nearly a tenth of a mile in a day. It travels unseen until, here and there, it comes to the surface as a spring, or perhaps is tapped to feed a well.
But mostly it contributes invisibly to streams, and so to rivers. And so pollution of the ground water is pollution of water everywhere. It must have been by such a dark underground sea that poisonous chemicals travelled from a manufacturing plant in Colorado to a farming district several miles away. What happened, in brief, is this. Eight years later, the facilities of the arsenal were leased to a private oil company for the production of insecticides.
Even before the changeover, however, mysterious reports had begun to come in. Farmers several miles from the plant reported unexplained sickness among livestock, and they complained of extensive crop damage; foliage turned yellow, plants failed to mature, and many crops were killed outright.
And there were reports of human illness. The waters used for the irrigation of these farms were derived from shallow wells. In , a study was undertaken, in which several state and federal agencies participated, and when the well waters were examined they were found to contain an assortment of chemicals. Such wastes as chlorides, chlorates, salts of phosphonic acid, fluorides, and arsenic had been discharged from the Rocky Mountain Arsenal during the years of its operation by the Army Chemical Corps.
The investigators knew of no way to contain the contamination—to halt its advance. All this was bad enough, but the most mysterious and probably, in the long run, the most significant feature of the whole episode was the discovery of 2,4-D in the holding ponds of the arsenal, even though no 2,4-D had been manufactured there during any stage of operations.
After long and careful study, the chemists at the plant concluded that the 2,4-D had been formed spontaneously in the holding ponds, from other substances discharged from the arsenal; in the presence of catalyzing air and sunlight, and quite without the intervention of human chemists, the ponds had become laboratories for the production of a new chemical.
Indeed, one of the most alarming aspects of the chemical pollution of water is the fact that in river or lake or reservoir—or, for that matter, in the glass of water served at your dinner table—are mingled chemicals that no responsible chemist would think of combining in his laboratory. The possible interactions between these chemicals, often comparatively harmless in themselves, are deeply disturbing to officials of the United States Public Health Service.
The reactions may take place between two or more chemicals, or between various chemicals and radioactive wastes. Under the impact of ionizing radiation, rearrangements of atoms could easily occur, changing the nature of the chemical in a wholly unpredictable way, and one that would be wholly beyond control. A striking example of the contamination of surface waters seems to be building up in the National Wildlife Refuges at Tule Lake and Lower Klamath Lake, both in California.
These refuges are part of a group, which also includes the refuge on Upper Klamath Lake, just over the border in Oregon. The three are linked, perhaps fatefully, by a shared water supply, and they lie like small islands in a great sea of surrounding farmlands—land reclaimed by drainage and stream diversion from an original waterfowl paradise of marsh and open water.
These farmlands around the refuges are now irrigated by water from Upper Klamath Lake. The irrigation waters, having been re-collected from the fields they have served, are pumped into Tule Lake and from there into Lower Klamath Lake. In the summer of , biologists picked up hundreds of dead and dying birds at Tule Lake and Lower Klamath Lake.
Most of them were fish-eating species—herons, pelicans, grebes, gulls. Upon analysis, they were found to contain insecticide residues identified as the chlorinated hydrocarbons toxaphene, DDD, and DDE. Fish from the lakes were also found to contain the insecticide residues; so were samples of plankton.
It appears that pesticide residues are now building up in the waters of these refuges, being conveyed there by return irrigation flow from heavily sprayed agricultural lands.
The refuges are critically important to the conservation of Western waterfowl. They lie in a strip of territory corresponding to the narrow neck of a funnel, in which all the migratory paths constituting what is known as the Pacific Flyway converge.
During the fall migration, the three refuges receive many millions of ducks and geese, from nesting grounds that extend from the shores of the Bering Sea east to Hudson Bay—in fact, fully three-fourths of all the waterfowl that move south into or through the Pacific Coast states in autumn. During the summer, the refuges provide nesting areas for waterfowl, and especially for two endangered species, the redhead and the ruddy duck.
If the lakes and pools of these refuges become seriously contaminated, the damage to the waterfowl populations of the Far West could be irreparable. Water, of course, supports long chains of life—from the small-as-dust green cells of the drifting plant plankton, through the minute water fleas, to the fish that strain plankton from the water and are, in turn, eaten by other fish or by birds, mink, raccoons, and man himself, in an endless transfer of materials from life to life.
We know that the minerals necessary for all these forms of life are extracted from the water and passed from link to link of the food chains. Can we suppose that poisons we introduce into water will not follow the same course?
The answer is to be found in the recent history of Clear Lake, California. Clear Lake lies in mountainous country some ninety miles north of San Francisco and has long been popular with anglers. The name is plainly inappropriate; actually the lake is rather turbid, because its bottom, which is shallow, is covered with soft black ooze.
Unfortunately for the fishermen and the resort dwellers on its shores, its waters have long provided an ideal habitat for a small gnat, Chaoborus astictopus. Although the gnat is closely related to mosquitoes, it is not a bloodsucker; indeed, it probably does not feed at all as an adult. However, the human beings who came to share its habitat found it annoying, because of its sheer numbers.
Efforts were made to control it, but they were largely fruitless until, in the late nineteen-forties, the chlorinated-hydrocarbon insecticides offered a new weapon.
The chemical chosen for a fresh attack was DDD, an insecticide that apparently offered fewer threats to fish life than DDT. The new control measures, undertaken in September of , were carefully planned, and few people would have supposed that any harm could result. The lake was surveyed, its volume was determined, and the insecticide was applied in the concentration of one part to every seventy million parts of water.
Control of the gnats was good at first, but by September of the treatment had to be repeated, and this time the chemical was added in the concentration of one part in fifty million parts of water. The destruction of the gnats was then thought to be virtually complete. The following winter months brought the first intimation that other life was affected; the western grebes on the lake began to die, and soon more than a hundred of them had been reported dead.
At Clear Lake, the western grebe is a breeding bird and also a winter visitant, attracted by the abundant fish of the lake. It is a bird of spectacular appearance and beguiling habits, building floating nests in shallow lakes of the western United States and western Canada. The newly hatched chick is clothed in soft gray down; only a few hours after emerging from the shell it takes to the water, riding on the back of the father or mother, nestled under the parental wing coverts.
Following a third assault on the ever-resilient gnat population, in September, —again in a concentration of one part of DDD to fifty million parts of water—more grebes died.
Both then and in , no evidence of infectious disease could be discovered on examination of the dead birds. But when someone thought of analyzing the fatty tissues of the grebes, they were found to be loaded with DDD in the extraordinary concentration of sixteen hundred parts per million. How could the chemical have built up to such prodigious levels?
The grebes, of course, are fish eaters. When the fish of Clear Lake were also analyzed, the picture began to take form: The poison had been picked up by the smallest organisms, concentrated, and passed on to the larger ones, which concentrated it further. Plankton organisms were found to contain about five parts per million of the insecticide; plankton-eating fish had built up accumulations ranging from forty to three hundred parts per million; carnivorous species of fish had stored the most of all.
One fish, a brown bullhead, had the astounding concentration of twenty-five hundred parts per million. It was a house-that-Jack-built sequence, in which the large carnivores had eaten the smaller carnivores, which had eaten the herbivores, which had eaten the plankton, which had absorbed the poison from the water. Even more extraordinary discoveries were made later.
No trace of DDD could be found in the water shortly after the last application of the chemical. But the poison had not really left the lake; it had merely gone into the fabric of the life that the lake supported. Twenty-three months after the chemical treatment had ceased, the plankton still contained as much as 5. In that interval of nearly two years, successive crops of plankton had flowered and faded away, but the poison had somehow passed from generation to generation.
And it lived on in the animal life of the lake as well. All fish, birds, and frogs examined a year after the chemical applications had ceased still contained DDD. The amount found in the flesh always exceeded by many times the original concentration in the water.
Among these living carriers were fish that had hatched nine months after the last application of DDD. California gulls had built up concentrations of more than two thousand parts per million. The grebes still carried heavy residues, and meanwhile their nesting colonies had dwindled, from more than a thousand pairs before the first insecticide treatment to about thirty pairs in Even the thirty seem to have nested in vain, for no young grebes have been observed on the lake since the last DDD application.
And what of the human being who has rigged his fishing tackle, caught a string of fish from the waters of Clear Lake, and taken them home to fry for supper? What could a heavy dose of DDD—and perhaps repeated heavy doses—do to him? In view of the evidence, the action seems a minimum safety measure. The thin layer of soil that forms a patchy covering over the continents controls our own existence and that of every other animal of the land. Without soil, land plants as we know them could not grow, and without plants no animal could survive.
Yet if our life depends on the soil, it is equally true that soil depends on life; its very origins and the maintenance of its true nature are intimately related to living plants and animals.
For soil is in part a creation of life, born of a marvellous interaction of life and inert matter aeons ago. The parent materials were gathered together as volcanoes poured them out in fiery streams, as waters running over the bare rocks of the continents wore away even the hardest granite, and as the chisels of frost and ice split and shattered the rocks. Then living things began to work their creative magic, and little by little these inert materials became soil.
Mosses took hold in these little pockets of simple soil—soil formed by crumbling bits of lichen, by the husks of minute insect life, by the debris of a fauna beginning its emergence from the sea. And not only did life help form the soil but living things now exist within it in incredible abundance and diversity; if this were not so, the soil would be a dead and sterile thing.
The soil exists in a state of constant change, taking part in cycles that have no beginning and no end. New materials are constantly being contributed as rocks disintegrate, as organic matter decays, and as nitrogen and other gases are brought down in rain from the skies. Simultaneously, materials are being taken away, harrowed temporarily for use by living creatures.
Subtle and vastly important chemical changes are constantly in progress, converting elements derived from air and water into forms suitable for the support of plant life, and in all these changes living organisms are active agents.
There are few studies more fascinating, and at the same time more neglected, than the study of the teeming populations that exist in the dark realms of the soil. We know too little of the links that bind the soil organisms to each other, to their world, and to the world above. Perhaps the most essential organisms in the soil are the smallest—the invisible hosts of bacteria and of threadlike fungi.
Statistics of their abundance take us at once into astronomical figures. A teaspoonful of topsoil may contain billions of bacteria. In spite of their minute size, the combined weight of bacteria in the top foot of a single acre of fertile soil, which itself weighs from ten to seventeen tons, may be as much as a thousand pounds. Ray fungi, growing in long filaments, are somewhat less numerous than the bacteria, but since they are larger, their total weight in a given amount of soil may be about the same.
With small, green cells of algae, these make up the microscopic plant life of the soil. Bacteria, fungi, and algae are the principal agents of decay, reducing plant and animal residues to their component materials. The vast cyclic movements of chemical elements, such as carbon and nitrogen, through soil and air and living tissue could not proceed without these microplants.
Without the nitrogen-fixing bacteria, for example, plants would starve for want of nitrogen, though they are surrounded by nitrogen-containing air. Other soil organisms form carbon dioxide, which on being dissolved in water becomes carbonic acid and aids in dissolving rock. Still other soil microbes perform the various oxidations and reductions by which minerals such as iron, manganese, and sulphur are transformed and made available to plants.
Also present in prodigious numbers in the soil are microscopic mites and primitive, wingless insects called springtails. Small as they are, both play an important part in breaking down the residues of plants, and thus aid in the slow conversion of the litter of the forest floor to soil.
The specialization of some of these minute creatures for their task is almost incredible. Several species of mites, for example, can begin life only within needles that have fallen from a spruce tree. Sheltered there, they digest out the inner tissues of the needle. By the time the mites have completed their development, only the outer layer of cells remains.
The truly staggering task of dealing with the tremendous amount of plant material in the annual leaf fall belongs to some of the small insects of the soil and the forest floor. They macerate and digest the leaves, and help to mix the decomposed matter with the surface soil. Besides all this horde of minute but ceaselessly toiling creatures, there are, of course, many larger forms, for soil life runs the gamut from bacteria to mammals.
Some of these larger forms are permanent residents of the dark, subsurface layers; some hibernate or spend certain parts of their life cycles in underground chambers; some come and go freely between their burrows and the upper world. In general, the effect of all this habitation of the soil is to aerate it and to improve both its drainage and the penetration of water throughout the layers of plant growth.
Of all the larger inhabitants of the soil, probably none is more important than the earthworm. At the same time, they draw quantities of organic matter contained in leaves and grass—as much as twenty pounds to the square yard in six months—down into the burrows, where they become part of the soil. This is by no means all they do. Their burrows aerate the soil, keep it well drained, and aid the penetration of plant roots.
And organic matter is broken down as it passes through their digestive tracts, so the soil is enriched by their excretory products. Is it reasonable to suppose that a so-called broad-spectrum insecticide can kill the burrowing larval stages of a crop-destroying insect without also killing the insects whose function may be the essential one of breaking down organic matter?
Or can we use a non-specific fungicide in orchards without also killing the fungi that inhabit the roots of many trees and aid the tree in extracting nutrients from the soil? The plain truth is that this critically important subject of the ecology of the soil has been largely neglected even by scientists and almost completely ignored by control men. The chemical control of insects seems to have proceeded on the assumption that the soil could and would sustain any amount of insult without striking back.
These synergists are sesamin, sesamolin, piperonyl butoxide, MGK bicycloheptenedicarboximide and sesamex. Piperonyl butoxide is perhaps the most widely used synthetic pyrethrin synergist. The insecticide activity of pyrethrins increases tenfold when 1 part piperonyl butoxide is mixed with 9 parts pyrethrin. There are no reports available on toxic effects on humans resulting from the exposure to piperonyl butoxide.
Adapted from: Klaassen, C. Add a badge to your website or intranet so your workers can quickly find answers to their health and safety questions.
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Search all fact sheets: Search. Type a word, a phrase, or ask a question. DDT is considered a possible human carcinogen. What countries still use DDT? By far the largest amounts are produced in India for the purpose of disease vector control. What is a fable for tomorrow about? Published in , the town described in the essay initially epitomizes the small towns of another time when everything seemed in perfect harmony with nature.
How do Carson tone style and purpose? Carson's tone, style, and purpose all change from a more imagery-based style in paragraphs to exposition. It draws the reader in, makes them curious, and then makes them recognize the potential tragedies that humans cause, and that gives the reader motive to read Carson's explanation. Is fed little tranquilizing pills of half truth?
Carson says that the public "is fed little tranquilizing pills of half truth" when it contests the use of pesticides. This metaphor is effective because of the truth behind it. Why is the book called Silent Spring? Is Carson opposed to the use of all toxins? Carson suggest that herbicides and other chemical insecticides be called biocides because instead of attacking the insects and plants, it is attacking all forms of life as well.
They should not be called 'insecticides', but 'biocides'.
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