In order to represent the range of early toxicological analysis of DDT, I have examined many of the early investigations of target insects, laboratory animals, wildlife, and humans. Because DDT and other chlorinated hydrocarbons (the class of synthetic compounds to which DDT belongs) had been developed as insecticides just before the U.S. entered World War II, scientists had little experience with the various forms (emulsions, dusts, solutions), doses (1 percent, 5 percent, 10 percent), or concentrations (5 pounds/acre, 1 pound/acre, 0.1 pound/acre) and did not know which would be the most effective in controlling insects without posing risks to other organisms, including plants. Since all of these factors needed to be determined, most studies tested various forms, doses, and concentrations.
In examining these studies, I have sought to capture the complexity of the experiments as well as the considerable variation in the type of DDT employed. Because of the inherent uncertainties in the forms of DDT just noted, and the variety of organisms involved, there was very little continuity linking one study to another. Different organizations were involved in the analysis of DDT, and some had competing agendas, which further complicated matters. Although these factors render the following analysis of preliminary studies of DDT somewhat disconnected, in the aggregate the studies marked a crucial episode in the history of toxicology and environmental risk.
DDT was initially synthesized by an Austrian chemistry student—Othmar Zeidler—in 1873. Zeidler was interested only in the process of synthesizing DDT, not in its potential uses. Not until 1939 did Paul Müller, a staff scientist at the Geigy firm in Basel, Switzerland, discover that DDT was extraordinarily powerful at killing insects on contact with long-lasting effects after application.2 As historian John H. Perkins has noted, seven criteria for distinguishing a “desirable commercial product” guided Müller’s research:
1. Great toxicity toward insects.
2. Rapid action, that is, onset of the paralysis within a few minutes.
3. Zero or weak toxicity toward warm-blooded animals, as well as toward fish and plants.
4. Absence of irritating action to warm-blooded animals, and little or no unpleasant smell.
5. Polyvalent action extending to the greatest possible number of insects.
6. Long duration of action, that is, great chemical stability.
7. Low price = economic advantage.3
Müller found that DDT exhibited all of the qualities with the exception of the second, “rapid action.” It could take hours for the insect to feel the lethal effects of the new insecticide. In a comparison with other insecticides, including nicotine, rotenone, pyrethrum, thiocyanates, and phenothiazines, Müller discovered that only DDT met the criteria of long duration of action. DDT also had considerable economic advantage over two of its primary competitors, rotenone and pyrethrum.4 The toxicity of the substance to animals and fish was a subject for further investigation. Geigy circulated information regarding an unspecified “new insecticide” to its subsidiary firms within two years of Müller’s discovery, but the U.S. subsidiary ignored the information. But after Geigy sent samples of DDT to the subsidiary, it recognized its considerable potential and forwarded samples to the USDA, which had been investigating new insecticides since 1940 as part of the war effort, under the supervision of Percy Nichol Annand, who had become chief of the Bureau of Entomology and Plant Quarantine (BEPQ) late in 1941.5
The BEPQ, under the auspices of the USDA, conducted the first tests of DDT in America.6 Founded in 1872 to develop control methods for boll weevil and gypsy moth populations, the BEPQ originally explored biological control methods. Under Leland Ossian Howard’s directorship (1894–1927), the BEPQ shifted to chemical control methods using such well-known insecticides as lead arsenate, Paris green, sulfur, and kerosene.7 The bureau’s laboratory in Orlando, Florida, directed by Edward F. Knipling, conducted many of the initial tests of efficacy and toxicity on DDT on the behalf of the U.S. armed forces. Knipling (1909–2000) was a young economic entomologist who had held various posts with the USDA in Iowa, Georgia, Texas, and Oregon, before he became director of the Orlando laboratory. Under Knipling’s direction, the Orlando laboratory tested the toxicity of DDT against several kinds of lice, including body louse (Pediculus humanus corporis), crab louse, and head louse in December 1942. By then DDT research had reached new levels of urgency as conditions in Naples, Italy, had reached a state of emergency as typhus threatened the city. Typhus, a deadly disease that thrives in crowded conditions, was carried by body lice.
Raymond C. Bushland (1910–1995) and his staff from the Sanitary Corps of the U.S. Army developed methods for testing various insecticides including DDT on body lice. First, they performed simple tests on approximately 7,500 chemicals in which they exposed lice to chemicals on treated cloths placed in small beakers. Based on the results, the BEPQ scientists eliminated all but a few chemicals from consideration. In the second procedure, utilizing the reduced set of chemicals, they placed the chemically treated cloths on the arms and legs of fully informed volunteers. In the final experiment, volunteers dressed as soldiers, wearing wool winter underwear that was treated with a promising chemical in powdered form. The researchers placed several hundred lice in various stages of maturity and over a thousand eggs directly on the volunteers’ clothing. One challenge of this research was maintaining colonies of thousands of lice requiring regular feedings. Also, volunteers had to endure the discomfort of feeding up to fifty thousand body lice once every two weeks. These tests revealed the value of two chemicals: MYL louse powder (made from pyrethrum) and DDT louse powder.
Bushland noted that DDT was effective in louse powder and proved to be “highly effective and longer lasting than any other louse treatment known to be in use.”8 DDT was similarly effective when impregnated in clothing. It remained effective for three to five weeks when clothing was not washed, and once-a-week washings reduced effectiveness to two or three weeks. DDT also proved effective against head and crab lice, particularly in a liquid preparation. After the FDA certified DDT powder as safe for humans, the BEPQ recommended it to the armed forces as a safe and effective louse powder. Historian David Kinkela has shown that efforts at the Orlando laboratory were augmented by a lab at the Rockefeller Foundation in New York. Between 1942 and 1944, Rockefeller scientists tested DDT on a range of subjects, including medical students in New York, conscientious objectors in New Hampshire, and an unwitting civilian population in Mexico.9 The extensive use of DDT by the armed forces (in cooperation with the Rockefeller Foundation) helped to prevent a potential typhus epidemic.10 Several years later, Knipling predicted even greater achievements: “We now believe that DDT provides the means of not only controlling lice and typhus but of eventually eradicating typhus from the earth.”11
Early testing at the Orlando laboratory demonstrated the efficacy of DDT against numerous other insects.12 Repeatedly, DDT’s property of persistence, lacking in most if not all of the other synthetic insecticides, proved to be important in insect control. For example, bedbugs posed a challenge to economic entomologists because both the common species (Cimex lectularius) and a tropical species (C. hemipterus) remained concealed and protected when insecticides were sprayed in a building. Entomologists at the Orlando laboratory found that DDT in various solutions of kerosene produced 100 percent mortality for bedbugs at least two months after application, and the strongest solutions completely eliminated them for more than nine months.13 Likewise, DDT spray controlled ticks both in the wild and on dogs. Researchers also noted that none of the experimental dogs showed any ill effects from the treatment.14
Even roaches could not withstand the effects of DDT. Researchers applied various spray and powder solutions of DDT to army mess halls that were heavily infested with German cockroaches (Blattella germanica). Within thirty minutes of spraying, 2,000 cockroaches lay dead and dying on the floor of one building. After twenty-four hours, scientists counted more than 4,000 nymphs (juveniles) and adults on the floor of each of three buildings. Thi
s study suggested that the virtually invisible residue was sufficiently toxic to protect buildings from German cockroaches for just a few days.15
Laboratory tests of the toxicity of DDT to twenty different insect pests, including blister beetle, Colorado potato beetle, imported cabbage worm, and several weevils, aphids, and termites, yielded 100 percent mortality in most cases using either a spray (1 pound per 100 gallons of water) or a 3 percent dust of DDT. Only termites evaded these deadly effects. Yet DDT seemed to repel the termites, which desiccated on the surface rather than passing through the DDT and sand mixture.16
Like termites, aphids proved resistant to DDT. Scientists recorded aphid mortality of a mere 5 percent with DDT, whereas nicotine destroyed 86 percent of the aphids. Dusting wet plants with DDT did not injure the plants (one of the problems associated with heavy metal insecticides).17 Other insects, including the cotton leafworm (Alabama argilacea) and the boll weevil (Anthonomus grandis), thrived in spite of DDT.18 One particularly refined study demonstrated that DDT could encourage population growth in certain insect species. In DDT-treated plots, the yellow sugar cane aphid population exploded to six times that of plots left untreated or plots treated with synthetic cryolite. The entomologists concluded, “It is apparent that the great increase in aphids resulting from the use of DDT would be a serious drawback to its use in control of the sugarcane borer.”19
In addition to nuisance insects like flies, ticks, bedbugs, and cockroaches, economic entomologists sought to determine the efficacy of DDT on the insects that threatened important economic crops, such as cotton and tobacco. To that end, they tested DDT on numerous cotton insects, but it proved to be far less effective against these insects than nicotine.20 Many scientists noted that in the field, the fruit and foliage of peaches, plums, or grapes were apparently not injured by DDT use. In a comparative study of insecticides, a team of scientists noted the advantages of DDT over other insecticides: “It remains effective for a longer period than derris and has the advantage over the lime and the lead arsenate sprays in that it leaves no conspicuous residue on the fruit and foliage.”21
Despite the lack of any apparent effect on plants, several researchers found ancillary effects on nontarget insects. Like aphids and cotton insects, the European red mite (Paratetranychus pilosus) became noticeably more abundant on trees sprayed with DDT. Researchers noted that lady beetles were absent from the infested plots, and their return brought the mites under control. Of course, DDT destroyed lady beetles. Similarly, large populations of the common red spider (Tetranychus spp.) ran rampant on the apples and under the bark of all trees sprayed with DDT, though red spiders were difficult to find in other plots.22 A few scientists expressed concern over the possible threat of DDT to beneficial organisms. One clearly deleterious effect was the destruction of lady beetles and the ensuing proliferation of cotton aphids. Worried about the impact on honeybees, they discovered that DDT, indeed, poisoned honeybees at 1 percent concentration and even lower.23
DDT appeared to be a wonder insecticide, able to control a wide range of insects (i.e., it was a broad spectrum insecticide). Underneath the shouts of praise, however, there were murmurs of concern. DDT did not seem to affect larger organisms or plants, but certain invertebrates survived its effects and proliferated as DDT destroyed their predators. Equally disturbing was the possibility that certain insects developed resistance to DDT’s effects. The toxicity of DDT to the Mexican fruit fly (Anastrepha ludens) proved to be highly erratic (more so than any other compound tested on it). All flies initially exposed to DDT in concentrations ranging from 2 to 4 pounds were killed, but after an extended period a small number of flies that immigrated into the study site withstood exposure to DDT at all concentrations up to 8 pounds. In contrast, 2 pounds of tartar emetic was considerably more toxic than any of the DDT concentrations tested.24
After notice from the Orlando laboratory of the BEPQ, the Tennessee Valley Authority through its Health and Safety Department inaugurated another important DDT testing program in 1943.25 This department was prepared for the study by its extensive experience in spraying for malaria control in the southeastern states. During the summer of 1943, the TVA conducted field tests to determine the practicability of DDT larvicidal dusts for the control of Anopheles quadrimaculatus (which, along with various Aedes mosquitoes, carried the malaria plasmodium). After several joint meetings with the technical staffs of the Orlando laboratory, the TVA expanded its research program to include laboratory and field studies on house spraying and to investigate the effectiveness of DDT as an anopheline larvicide (killed juveniles) and adulticide (killed adults) when distributed by airplane as a dust, a spray, or a thermal aerosol.
With the advice and assistance of scientists from government agencies and universities, the TVA tested the new insecticide extensively. The tests fell into several categories: acute toxicity, residual toxicity, repellency of DDT-treated surfaces, use of DDT as larvicide (distribution by boat and air), DDT as an adulticide, and toxicity with fish and fish food organisms. Several interesting findings emerged from this research. First, a comparison with pyrethrum (a pesticide created from the stigmas of chrysanthemum flowers) showed DDT to be nearly an order of magnitude less toxic to male and female mosquitoes than pyrethrum, which was the primary alternative in mosquito control. However, DDT showed very high residual toxicity (barns treated with 200 mg DDT/square foot remained almost entirely free of mosquitoes for eleven weeks, while occupied houses treated with 250 mg DDT/square foot remained free of A. quadrimaculatus for at least three months). Still more striking was the discovery of an even higher degree of residual toxicity to house flies. Finally, scientists found that DDT dusts and thermal aerosols gave no evidence of injury to fish or other aquatic organisms when applied by plane at rates of 0.1 pound per acre. A 5 percent solution of DDT in kerosene applied at approximately 0.25 pound DDT per acre, however, largely destroyed aquatic insects living in close contact with the water surface (notably Hemiptera and Coleoptera).26
From the results of this early research into the effects of DDT on mosquitoes, it was possible to conclude that the new insecticide was not the most toxic of available insecticides (pyrethrum appeared to be much more toxic to Anopheles mosquitoes according to early tests). Nevertheless, DDT showed great promise as a “magic bullet” in the battle against mosquitoes, malaria, and other diseases. The new insecticide destroyed 90 percent of all Anopheles mosquitoes in many of the tests, whether conducted indoors or outdoors. That figure was impressive to economic entomologists, but the difference between 90 percent and 100 percent would become extremely significant when DDT became available for widespread use.27 Along with high toxicity, DDT showed a high level of persistence in most environments (another factor that impressed economic entomologists).
Beyond the toxicity of DDT to target organisms (e.g., Anopheles quadrimaculatus), scientists at the TVA considered potential side effects, such as the impact on nontarget insects and fish. At least as significant as the study of nontarget organisms was the range of environments examined. TVA scientists evaluated the effect of DDT in houses (occupied and unoccupied), fields and forests, and bodies of water.
Percy N. Annand (chief of the BEPQ) drew heavily on the above experiments in his address “How about DDT?” to the annual meeting of the National Audubon Society in October 1945, in New York City. Annand argued that insects posed a serious threat to forests in the United States and that he had witnessed the destruction of more than three billion board feet of lumber wrought by the spruce bark beetle in Colorado over the span of three years. After listing many of the insects investigated in the studies by the bureau and other agencies, Annand acknowledged that DDT produced unsatisfactory results against other species, but on the whole, he downplayed potential risks associated with DDT. He suggested that it could be applied annually for many years before harmful quantities would build up in the soil, even if it did not decompose. As for the toxicity of DDT, Annand noted that the FDA would set the tolerance in
fruits for DDT at 7 mg/kg, which was similar to the tolerance in fruit for lead and fluorine.28
Annand recognized the need for more research on the toxicity of DDT to farm animals and noted that tests were under way at several experimental research stations. The threat to honey bees concerned him, but compared to the effects to arsenicals, he thought DDT would be considerably less toxic to bees and that much of the early alarm was unwarranted. Likewise, Annand dismissed concerns about insect parasites and predators: “Except in operations far more extensive than any which are contemplated, it would appear that, even though all the parasites in an area were killed, it soon would be repopulated by infiltration from the outside.”29 Moreover, the chief of the Bureau of Entomology believed that the desire to maintain the balance of nature was unrealistic: “As a matter of fact, there are probably very few cases in which nature is balanced, and certainly it is grossly out of balance when there are extensive outbreaks of insect pests.”30
After exploring the effects of DDT on wildlife, Annand concluded optimistically: “As our experimental work progresses, we are much impressed by the very small amounts of DDT that are effective against injurious insects. In the case of forest insects, particularly defoliators, excellent control has been obtained with ¼ to 1 pound of DDT per acre. The gypsy moth, cankerworms, and sawflies seem particularly easy to control.”31 Thus what distinguished DDT from other insecticides, such as pyrethrum and lead arsenate, was the remarkably low amounts needed to control target insects. In Annand’s view, these quantities were too low to pose risks to other wildlife, whereas one had to apply much greater quantities of other non-synthetic insecticides to control insects effectively. By extension, Annand also believed that DDT could be applied at low rates and that it would accumulate much more slowly in the environment. Such a statement can be seen as comparative (earlier forms of insecticides accumulated in the environment rapidly). In his statements to the National Audubon Society, Annand clearly distilled the hopes and expectations of economic entomologists and public health officials for DDT.
Banned Page 7
