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  Because of its potential use in dairy barns, an early concern was whether or not DDT was eliminated in milk. One study addressed this concern through a series of experiments involving rats and goats. Researchers fed a mixture of 0.1 percent DDT in chicken mash to three young female rats, each of which was nursing a one-day-old litter. Typical DDT tremors appeared in the adult rats between days 6 and 13 and in the young between days 14 and 15. After day 18 all of the rats had died except one adult and one juvenile. In another experiment, nine adult rats fed solely on goat’s milk received daily oral dosages of 1 gram DDT per 8 to 9 pounds (3.63 to 4.08 kg) body weight. All the rats died after 2 to 29 days with symptoms of DDT poisoning. Still another experiment demonstrated that nursing rats developed symptoms of DDT poisoning shortly after their mothers began to feed only on milk from the goats regularly fed DDT. A kitten also died with typical DDT symptoms after consuming the milk of a goat fed DDT for 25 days, but an unweaned baby goat showed no signs of poisoning despite freely suckling from a goat under treatment for 27 days. Researchers also wondered if the milk of goats regularly sprayed with DDT would become contaminated. In light of these mixed findings, researchers urged caution, particularly regarding the milk of cows: “The data strongly suggest the need for more intensive research on the toxicity of milk from dairy cows ingesting DDT residues either from sprayed or dusted forage plants or from licking themselves after being sprayed or dusted with this insecticide.”53

  The precautionary principle would suggest that those who stand to profit from the sale of a good like a chemical insecticide should demonstrate its safety before releasing it to consumers. Lack of premarket testing (except for target organisms) left regulatory agencies like the FDA and PHS scrambling to determine the toxicity of DDT.54 Laboratory tests raised numerous issues that called for further study. Although the acute toxicity of DDT was, as expected, quite low, chronic toxicity experiments suggested that in the long term, DDT could pose serious threats. Certainly the findings of the FDA pharmacologists in the Division of Pharmacology, drawn from extensive studies that addressed acute and chronic toxicity, effects on various tissues, feeding and inhalation exposures, as well as metabolic function, concentration in fat cells and milk, significantly undermined the contention that DDT was completely harmless to warm-blooded animals. There was enough ambiguity in their findings, however, to suggest that DDT used judiciously did not pose a great threat to mammals. Early wildlife studies were no more conclusive.

  The path from the laboratory to the field was fairly direct. During spring 1945 scientists began to examine wild animals for the effects of DDT. Researchers fed field mice various concentrations of DDT from 0.40 percent to 0.01 percent and 0 percent (as a control). Neither the control animals nor the mice receiving low doses (0.01 percent to 0.10 percent DDT) exhibited toxic effects. At 0.20 percent, however, two mice died before the end of the experiment (thirty days). Doubling that dosage killed four mice within nine days (a fifth mouse died on the twenty-first day). White-footed mice appeared less susceptible to DDT than field mice in similar tests. Inhalation experiments were less conclusive than the ingestion tests. Ten field mice placed in an artificial habitat sprayed by hand with a DDT oil mixture showed no evidence of toxicity even though DDT-sprayed oats were introduced on the seventeenth day.55

  Wild cottontail rabbits were also highly sensitive to DDT in the laboratory. Four rabbits fed crystalline DDT developed tremors. Two died (one on the fifteenth day and one on the twentieth day of the experiment). In another series of toxicity tests with cottontail rabbits, DDT was administered through a stomach tube in six dose levels from 500 to 2,500 mg/kg bodyweight. No symptoms appeared in rabbits exposed to levels below 1,500 mg/kg, but one rabbit at the 1,500 mg/kg level showed tremors in the second day (but recovered). At the other levels, two out of four rabbits at the 2,000 mg/kg level died on the third and thirteenth days, respectively, and two out of three rabbits at the 2,500 mg/kg level died on the seventh and twelfth days, respectively.56

  Like mammals, birds reflected the effects of DDT in laboratory studies. Don R. Coburn and Ray Treichler, biologists at the U.S. Fish and Wildlife Service, fed five-week-old bobwhite quail a mash diet containing DDT at percentages ranging from 0.40 percent to 0.005 percent. All experimental quail fed mash containing 0.05 percent or more of DDT died. Even at .025 percent, half of the bobwhite quail perished, and there were even deaths at the lowest percentage used in these experiments (.005 percent). To determine the acute toxicity of DDT to bobwhite quails, the FWS researchers administered single doses of DDT either in crystalline form or in a vegetable oil solution. For the crystalline form, dosages ranged from 50 mg/kg body weight to 1,000 mg/kg body weight, while the dosages for the oil solution ranged from 40 mg/kg bodyweight to 1,000 mg/kg body weight. These tests revealed that 200 mg/kg bodyweight was required to cause significant mortality. Unlike researchers with a background in pharmacology, the FWS biologists did not determine LD50s for quail, but they were able to establish rough estimates of acute toxicity. Finally, Coburn and Treichler noted that the symptoms of DDT poisoning found in other animals were the same in birds: excessive nervousness, loss of appetite, tremors, muscular twitching, and persistent rigidity of the leg muscles.57

  As a class, wild amphibians exhibited high levels of sensitivity to DDT. After collecting wood frog egg masses from bottomland ponds along the Patuxent River, Lucille Stickel, another biologist with the FWS, separated the tadpoles into separate aquarium jars at the rate of 100 per jar. Stickel left control jars untreated and treated others with oil only. A third group of jars received DDT and oil at a rate equivalent to 5 pounds per acre. All tadpoles treated with DDT died within three to five days, while the controls and tadpoles in jars treated with oil alone remained healthy.58

  Wild-caught fish exhibited slight if any reaction to DDT in early experiments. In one early study, Eugene Surber (another FWS biologist) stocked 100 brook trout, 100 rainbow trout, and 100 bluegill sunfish in mid-August in four connected raceways. Researchers sprayed with an oil solution of DDT at the rate of 1 pound per acre and observed that the DDT remained on the surface of the water for at least four hours. None of the brook trout or the rainbow trout died or showed signs of contamination, but 4 to 12 percent of the bluegill sunfish died within five days.59 One potential problem with the design of this study is that DDT in an oil solution may not have dissipated beyond the surface of the water, meaning that only the fish coming into regular contact with the surface would have been significantly exposed.

  Surber conducted another experiment, which specifically addressed this problem by stocking each of twelve small, hard-water ponds with fingerlings (50 bluegills and 50 largemouth bass). Three ponds received no treatment to serve as controls. Then he applied DDT in three forms to the other ponds: in an oil solution, an emulsion, and a suspension. After one week, FWS researchers drained all of the ponds and counted the surviving fish. DDT in suspension killed very few fish while DDT in solution killed 50 to 60 percent of the bluegills but very few bass. DDT in emulsion killed all fish of both species. In yet another experiment, Surber tried to determine if fish would be killed when fed only on DDT-contaminated flies. With his colleagues, he stocked three ponds with 25 adult and 25 fingerling bluegill sunfish. Fish in all three ponds could gorge themselves on flies. In two of the ponds the flies had been sprayed with a 12 percent solution of DDT at the rate of 1 pound per acre, and in the third pond the flies were untreated (as a control). None of the fish in any of the ponds died.60 The impossibility of recording symptoms in fish, other than death, however, prevented these experiments from detecting subtler effects of chronic DDT poisoning.

  Several conclusions emerge from the laboratory tests conducted on wild animals. With the exception of the tests on field mice and rabbits (both groups with lab analogies), they bear little resemblance to the tests conducted on lab animals. Experiments with birds, fish, and amphibians had to be created, de novo. On the whole, the size of the samples was significantly
lower in laboratory studies of wildlife. While lab scientists noted considerable variation in individual susceptibility to the toxicity of DDT, wildlife biologists began to track considerable variation in the susceptibility of the many species of wildlife. These factors would all intensify as wildlife biologists moved from the laboratory to the field to study the effects of DDT.

  In 1946, two scientists with the FWS, Clarence Cottam (assistant director) and Elmer Higgins (chief of the Division of Fishery Biology), captured the expectations for, and fears of, DDT in one of the first reviews of its effects on fish and wildlife (most of which had not been published at the time of the review): “From the beginning of its wartime use as an insecticide the potency of DDT has been the cause of both enthusiasm and grave concern. Some have come to consider it a cure-all for insect pests; others are alarmed because of its potential harm. The experienced control worker realizes that DDT, like every other effective insecticide or rodenticide, is really a two edged sword; the more potent the poison, the more damage it is capable of doing.”61 In discussing the potential hazard of DDT to wildlife, Cottam and Higgins emphasized the concept of specificity in toxicity: “Most organic and mineral poisons are specific to a degree; they do not strike the innumerable animal and plant species with equal effectiveness; if these poisons did, the advantage of control of undesirable species would be more than offset by the detriment to desirable and beneficial forms. DDT is no exception to this rule. Certainly such an effective poison will destroy some beneficial insects, fishes, and wildlife.”62 But what risk did extensive use of DDT pose to wildlife?

  To determine the extent to which DDT contaminated the environment, scientists sprayed a DDT solution at the rate of 2 pounds of DDT per acre from an airplane on a 117-acre tract of well-drained forest on the Patuxent River bottomland in the Patuxent Research Refuge in Maryland. By placing petri dishes (four inches in diameter) throughout the area prior to spraying and running them through chemical analysis afterward, researchers discovered that the amount of DDT reaching the ground through the forest canopy represented a small fraction of the original deposition (0.008 pound per acre under tree canopy and ground cover, 0.5 pound per acre on open forest floor under tree canopy, and 0.6 pound per acre under tree cover along a riverbank). Insect control was not an objective of the study, but researchers noted that many insects died within a few days of application of the spray, particularly adult mosquitoes. The effects were only temporary; most species returned to normal numbers in two to three weeks.63

  Scientists at Patuxent Research Refuge (a National Wildlife Refuge) tracked the effects of DDT on mammals, birds, amphibians, and fish. Using 50 live traps, researchers counted the number of two species of insect-eating mammals (the short-tailed shrew and the deer mouse) on a 10-acre sprayed area and on a similar 10-acre control area just over a mile from the sprayed tract. The populations of both mammals declined in both sites, so the findings were without statistical significance: “The differences on the two areas are not of statistical significance, and the consistent reductions may be due to seasonal changes in behavior.”64 For the bird studies, FWS scientists conducted an intensive search for nests before they sprayed a 31-acre area within the 117-acre tract of bottomland forest. They also made censuses of two additional areas: a 22-acre area adjacent to the sprayed area and a 32-acre area slightly more than a mile away. Only one species, the American redstart (Setophaga ruticilla) declined significantly, possibly because as a tree-top feeder it suffered greater exposure.65

  Another experiment attempted to determine if spraying DDT at the rate of 5 pounds/acre to birds’ nests would disrupt the hatching of eggs, disturb the development of young, or cause the abandonment of eggs. Using a hand atomizer, scientists sprayed DDT on a one-square-foot area surrounding and including a nest. After attempting to compare pairs of nests of the same species, researchers concluded: “The treatment with DDT showed no detrimental effect on the hatching of eggs or on the development of the young; it caused no abandonment of nests even when they were located in such confined quarters as bird boxes.”66

  In the same sites as those used for the bird studies, another researcher studied the effects of DDT on frogs and toads in the wooded bottomland. In one experiment, various species of frog and toad tadpoles were stocked in open-topped cages and inspected daily for nine days after spraying. None of the animals were visibly affected by the spraying. In another experiment, a scientist treated two artificial ponds inhabited by adults and tadpoles of several species of frogs and toads with xylene and fuel oil only, two with DDT at 1 pound/acre in an oil solution, and two at 5 pounds/acre in an oil solution. Several untreated ponds served as controls. One pond of each pair was deeper than the other. Researchers sampled all of the ponds with dip nets twice prior to spraying and several times after. None of the amphibians in the deeper pond treated with 1 pound/acre DDT died, but several frogs and large-frog tadpoles as well as a young water snake died in the shallow pond (five inches deep at the center). Several additional frogs died in both of the ponds treated with 5 pounds/acre DDT. Nevertheless, some amphibians survived in all ponds. Researchers attributed all deaths to DDT.

  As in the laboratory experiments, fish were highly sensitive to DDT. Nearly a mile of the Patuxent River (a muddy stream with a flow of about 130 cubic feet/second during the summer) passed through the 117-acre tract that was sprayed with DDT. Only 9.5 hours after the initial spraying, researchers seined 95 dead fish out of the stream near the lower end of the sprayed section. Dead fish continued to drift into the stop net four days after treatment, but the greatest number were lost during the first two days. In what was in effect a controlled experiment, scientists stocked eight shallow, soft-water ponds (20-by-50 foot) with several fish species. After spraying the three groups of ponds with three concentrations of DDT (0.1, 0.5, and 1 pound/acre) and leaving one pond unsprayed as a control, researchers found that mortality was considerable in all ponds but most severe in the pond treated with 1 pound/acre.67

  The majority of the wildlife studies discussed so far had little or no connection with laboratory studies, except for the several studies of wildlife in the laboratory, which had made a concerted effort to mirror laboratory experiments. Overall, the techniques, language, and approach to experimental design shared little with the lab. The reasons for the marked differences between the lab and the field will emerge in a closer study of the major field studies of the effects of DDT on wildlife.

  As with the laboratory studies, several agencies independently evaluated the toxicity of DDT to wildlife. One of the most extensive analyses of the potential impact on nontarget organisms was undertaken by the PHS at the Carter Memorial Laboratory in Savannah, Georgia. Directed by Clarence M. Tarzwell, a retired senior assistant sanitarian at PHS, this study sought to explicate the effects of DDT mosquito larviciding on wildlife. More specifically, the purpose of the studies “was to determine at what dosages and in what manner or physical state DDT could be routinely used as an anopheline larvicide without being significantly harmful to other organisms of economic or recreational value.”68 In late 1944, the first year of the study, researchers conducted experiments on the effects of routine hand application of DDT dusts, emulsions, and solutions, varying the method of application, types of larvicides, and dosages of DDT. Early in the study, they discovered that tight emulsions and solutions of DDT applied at a rate of 0.4 pound/acre were deleterious to the fish population in shallow waters. For this reason, they shifted their emphasis to DDT dusts or solutions at lower concentrations (0.1, 0.05, or 0.025 pound/acre). At these levels, they observed no fish mortality in individual treatments, but repeated routine treatments caused fish to begin dying in the interval between three and ten treatments, and eleven to eighteen treatments at this rate significantly reduced the population.69

  By 1945 the PHS had significantly expanded its assessment of the toxicity of DDT. Researchers studied the effects of the routine treatment at 0.1 pound DDT per acre, applied by airplane to extensive area
s of the Savannah River National Wildlife Refuge in Georgia. The initial emphasis on fish and fish food sources (surface, bottom, and plankton organisms), expanded with the assistance of the FWS to include studies of the effects of routine treatments on amphibians, reptiles, birds, mammals, and terrestrial insects. Spraying continued during a third season (1946) as researchers considered the cumulative effects of two years of routine treatments on the fish and wildlife populations.70

  For this research, scientists examined ponds in three areas of the Savannah River Refuge and fourteen natural ponds at the Plant Introduction Laboratory of the Bureau of Plant Industry. They sprayed DDT at weekly intervals (routine treatments) in various concentrations ranging from 2 pounds to 0.025 pound per acre, but most experiments fell in the range of 0.1 to 0.025 pound per acre. Scientists used two methods to detect kills or changes in the population of surface organisms. The first method was gross observations taken 24 to 48 hours after treatment to detect any kill of the larger insect forms (such as Gyrinidae, Dytiscidae, Hydrophilidae and Corixidae). The second method was to take quantitative surface samples before and after treatment in order to determine any changes in the population of surface organisms. Researchers soon discovered a significant problem with their methodology. After collecting twenty-five random samples at the beginning of the study and before and after treatment, they realized that there were no large homogeneous areas suitable for such sampling in the ponds, and that the numbers of organisms found in the different samples varied considerably. Further, they noted that in most instances the variation was so great that it would have been impossible to detect even large differences due to treatment. Consequently, they abandoned random samples in favor of paired samples.