Banned
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The EPA was forced to apply FEPCA shortly after it was passed by Congress. The EDF initiated the legal action in 1968 (before FEPCA) by petitioning the FDA to reduce to zero all food tolerances for two closely related chlorinated hydrocarbons—aldrin and dieldrin—based on evidence from the Shell Chemical Company study indicating that the compounds caused cancer in mice. The day after EPA opened for business, EDF requested it to suspend and cancel all uses of aldrin and dieldrin. Ruckelshaus, the EPA administrator, complied by issuing a notice of intent to cancel all registrations of the two pesticides. The companies that held the registrations for aldrin and dieldrin demanded review by the National Academy of Science (NAS), which they could do under FIFRA. After nearly two years, the NAS report reached the conclusion that the pesticides appeared to pose no threat to human health even in their uses as a corn insecticide. EDF next requested that the EPA suspend uses of aldrin and dieldrin through the D.C. Circuit Court in 1972. Cancellation hearings for the two pesticides began in August 1973 after the passage of FEPCA. The new EPA administer, Russell Train, reviewed testimony generated over the course of twelve months (more than 35,000 pages). Train concluded that aldrin and dieldrin represented an “imminent hazard to man and the environment.” After an expedited suspension hearing, a judge recommended suspension of all uses. Train agreed to the proposal but Shell Chemical Company appealed, and in April 1975 the D.C. Circuit Court upheld the ruling.19 The cancellation of aldrin and dieldrin took place between 1968 and 1975, which meant that the process began under the rules of FIFRA and concluded under FEPCA.
As with the cancellation of DDT, cancelling aldrin and dieldrin proved to be a labored and tortuous process of requests to FDA then EPA, chemical company appeals, proposed rulings, additional appeals, and, eventually, resolution. As of 1975, the EPA had canceled registrations on several of the persistent chlorinated hydrocarbons that posed the most significant threats to the environment. Meanwhile, many other pesticides remained on the market, including the highly toxic organophosphates. And chemists strove to develop synthetic forms of naturally occurring insecticides.
Banning DDT and other chlorinated hydrocarbons along with some of the organophosphates in the early 1970s left economic entomologists, farmers, and public health officials with a significantly reduced palate of chemical insect control options. Recall from chapter 1 the discussion of insecticide options that preceded the development and proliferation of DDT and other synthetic insecticides. Before there was DDT, before there were heavy metal insecticides such as lead arsenate, there was pyrethrum. Pyrethrum was a potent naturally occurring insecticide, but production at the levels required to control insects on crops proved problematic. By the 1940s, chemists were committed to developing synthetic analogs to pyrethrum, the first of which—allethrin—saw the light of day in 1949. Along with its alcoholic component and other forms, allethrin served as an important alternative to the natural form of pyrethrum, when it was not available.20 But as we have seen, DDT, other chlorinated hydrocarbons, and the organophosphates surged onto the market and quickly dominated agricultural and public health insect control campaigns in the late 1940s and 1950s, leaving very limited utility for the first synthetic pyrethroid. John E. Casida, a chemist at the University of California–Berkeley, captured alletrin’s significant disadvantage to DDT to American consumers thusly: “Allethrin, the first synthetic pyrethroid, was useful for household pests if you accepted a many-step synthesis, whereas DDT controlled almost every pest and could be made in one or two steps at only a small fraction of the cost. No wonder billions of kilograms of chlorinated hydrocarbons including DDT were used!”21
In 1966 the synthesis of pyrethroid-like compounds took a significant leap forward through the efforts of Michael Elliott and a group of organic chemists at the Rothamsted Experimental Station in Harpenden, England. Elliott successfully produced the first compounds with properties superior to the properties of the natural ester, indicating potential as practical insecticides. The most valuable property of bioresmethrin was that it combined great insecticidal activity with very low mammalian toxicity, which led to the commercial production of resmethrin and bioresmethrin.
In 1970 Elliott presented his findings regarding the considerable potential of new synthetic pyrethroids at the International Conference on Alternative Insecticides for Vector Control, sponsored jointly by Emory University, CDC, and WHO, a conference that focused predominantly on so-called anticholinesterase insecticides, which is to say the organophosphates and carbamates, although pyrethroids and chlorinated hydrocarbons also received attention. To make the case for the potential of synthetic pyrethroids, Elliott compared the toxicity of bioresmethrin and parathion (still very much in use in 1970) (see table 2). Elliott interpreted the table noting that bioresmethrin was more toxic than parathion to houseflies but much less toxic to rats. To interpolate what must have been a stunning revelation to both economic entomologists and toxicologists, bioresmethrin was four times more toxic to houseflies (the target organism) than parathion and less toxic to rats by more than three orders of magnitude! Parathion was widely considered to be one of the most toxic chemicals known to man, and its toxicity applied equally well to mammals and birds as to insects. As an aside, Elliott noted that synthetic pyrethroids were “non-persistent,” which distinguished them from chlorinated hydrocarbons. Economic entomologists, however, viewed the blessing of rapid decomposition as a curse. Allethrin, bioallethrin, resmethrin, bioresmethrin, and others were all unstable in air and light, a property that significantly limited the utility of these compounds in the field, particularly against pests of agricultural crops, despite the manifest advantages of these chemicals (potency against many insect species, rapid action, and low mammalian toxicity). Elliott and the researchers at the Rothamsted Experimental Station endeavored to synthesize esters that were more stable in light by a factor ten to one hundred than previous pyrethroids, while retaining the strong activity against insects and the low toxicity to mammals.22
Table 2
Toxicity of Parathion and Bioresmethrin to Housefly and Rat
In 1972, Elliott and his colleagues discovered an “exceptionally valuable combination” of esters and alcoholic components. They called the new compound permethrin. It was more active against many insects than had been predicted from its components. Moreover, permethrin was more stable in air and light than other potent pyrethroids, and it had lower mammalian toxicity than other esters created from the same acid. Elliott would later conclude that permethrin was “stable enough to control insects in the field as efficiently as established organophosphates, carbamates, and organochlorines, many of which it surpassed in potency.”23
Elliott compared permethrin with the most significant chlorinated hydrocarbons, organophosphates, and carbamates that were the most popular insecticides. At 0.7 μg/g, The LD50 of permethrin to insects was lower than that for any other insecticide. Parathion showed the next lowest LD50 at 1 μg/g for insects. Parathion’s LD50 for rats was 11 mg/kg! In contrast, the LD50 for rats for permethrin exceeded 1,000 mg/kg. Of the dozen or so insecticides that Elliott included in the table, only malathion had an LD50 for rats that was greater (1,400 mg/kg). Nevertheless, permethrin stood apart from all the other insecticides on the basis of yet another metric: the ratio of LD50 of rats to insects, which provided an index to an insecticide’s relative effectiveness and safety.24
With such high toxicity to insects and such low toxicity to mammals, permethrin and other synthetic pyrethroids had great potential as agricultural insecticides. Laboratory and field tests indicated that permethrin could effectively control insects of various orders, including moths, mosquitoes, flies, and ants. Beyond its strong potential for uses in agriculture, permethrin showed a wide range of activity against veterinary parasites. Researchers found that the new insecticide killed 100 percent of cockroaches (Blatella germanica) over the course of twelve months, when applied to plywood in the amount of 300 mg/m2. Permethrin also showed promise in the control of dai
ry barn pests, as reflected in this statement: “Preliminary results indicate that in hand spray applications to the entire body surface of lactating dairy animals, at a level needed for adequate fly control, residues of permethrin in milk are unlikely to pose a problem.”25 Recall that the USDA officially discouraged comparable uses of DDT as early as 1949. As with other insecticides, and particularly in cases in which other insecticides had been used intensively, insects developed resistance to pyrethroids. After recommending caution with regard to the development of resistance as a result of over-enthusiastic application, Elliott optimistically concluded his review of the toxicity and potential for pyrethroids: “In favorable circumstances however, synthetic pyrethroids should help to control more insect pests in the future with smaller hazard to man and the environment than earlier, widely used pesticides.”26
The promise of synthetic pyrethroids continued to emerge as the insecticides were developed for agricultural and household use during the 1970s. Toxicological profiles for the chemicals still appeared to be quite favorable, especially in comparison with chlorinated hydrocarbons and organophosphates. Initial assessments of mammalian toxicity were borne out by subsequent studies. Avian toxicity also appeared to be quite low, but toxicity for fish was strikingly high (sensitivity ranged into the parts per billion). Toxicologists recommended caution to avoid the contamination of lakes and streams as well as commercial fisheries. Besides fish, honeybees were also highly susceptible nontarget organisms. Beneficial aquatic insects and crustaceans were also potentially vulnerable to pyrethroids.
As with other organic insecticides, resistance presented a potential problem. Neither organophosphates nor carbamates seemed to introduce cross-resistance to pyrethroids, but houseflies on Danish farms exhibited considerable cross-resistance to pyrethroids after developing resistance to DDT. Cross-resistance among pyrethroids was potentially serious. Selection with bioresmethrin of a pyrethroid-resistant field strain resulted in a resistance factor of 1,400-fold. This same strain revealed a resistance factor of 60,000-fold to decamethrin, despite no previous exposure.27
In 1978, Michael Elliott reviewed the status of insecticide development. He argued that conventional insecticides were necessary to protect food supplies and other agricultural crops and to control disease. The only two natural insecticides that had significant histories of use were pyrethrum and nicotine, derived from the leaves of plants in the nightshade family (Solanaceae). Elliott noted that nicotine was still used in surprisingly large quantities. His surprise may have followed from the insecticide’s variable toxicity to insects. Nicotine was much more toxic to silkworms than houseflies, for example. More likely, the continued use of nicotine surprised Elliott because it was “one of the most rapidly acting and deadly poisons known against man.”28 Despite efforts to find chemical analogs to nicotine, none were less toxic to humans. Nevertheless, agricultural chemists continued their efforts to develop synthetic nicotine analogs. In contrast, rotenone, another naturally occurring pesticide extracted from the roots and stems of several tropical plants, had a low toxicity to mammals, but like nicotine it was far more toxic to certain insects, for example the mustard beetle, than to the housefly or the honeybee. And, like nicotine, no useful chemical analogs had been synthesized. Mammein (extracted from the seeds of Mammea americana), was another natural insecticide, but no synthetic analogs emerged. Elliott reviewed and dismissed other natural insecticides before turning to the synthetic insecticides.29
After briefly reviewing the history of the introduction of DDT and the chlorinated hydrocarbons, as well as of the organophosphates and the carbamates, Elliott noted that no other major groups of insecticides had been developed. Then he turned to a discussion of the synthetic pyrethroids and how he and his colleagues at Rothamsted had developed the new class of insecticides. As in his other papers, Elliott noted that the synthetic pyrethroids represented some of the most toxic insecticides, while presenting remarkably little risk to mammals. To illustrate this point, he listed a range of insecticides and other poisons in order of median effective doses to mammals and insects. Like other, similar charts, this one supported Elliott’s fundamental argument, but he extended its implication: “Pyrethroids therefore constitute a broad class of lipophilic insecticides which promise to complement the more polar organophosphates and carbamates and to replace the organochlorine compounds for certain applications.”30
To underscore the low toxicity of pyrethroids to mammals, Elliott expanded the chart, presented earlier in the paper, that showed toxicity to insects and mammals. As in other analyses, pyrethroids appeared among the least toxic insecticides to mammals and among the most toxic to insects. Recommended rates of application for pyrethroids were also much lower than rates for organochlorines, organophosphates, and carbamates. The low rates of application meant that pyrethroids would produce less environmental contamination than organochlorines even if pyrethroids shared persistence with the organochlorines. But the nonvolatile pyrethroids broke down rapidly in soil, which reduced the likelihood that appreciable residues would accumulate.31 By way of conclusion, Elliott emphasized the key similarity between the organophosphates, the carbamates, and the pyrethroids: “The greatest number of practical compounds has been developed in the classes where the groups attached to a central function can be varied widely, to give a range of diverse, but related compounds. The organophosphates, carbamates and pyrethroids are all of this type. The more active and stable pyrethroids have extended the range of useful insecticides.”32 Despite such optimistic claims regarding the synthetic pyrethroids, the ban of chlorinated hydrocarbons like DDT, dieldrin, and aldrin left farmers with limited viable options for pest control.
In Silent Spring, Carson recommended judicious use of insecticides and the exploration of alternative methods, such as biological control. To what extent did farmers heed Carson’s recommendation? How did regulatory intervention shape use patterns in the aftermath of the ban on DDT and organochlorines? Such questions return us to one of the prominent themes of this book: namely, that throughout the twentieth century, Americans demanded insect control to boost agricultural productivity and protect against insect-borne disease. It is possible to track insecticide use in the United States at five-year intervals between 1966 and 2002, and patterns in insecticide use can provide partial answers to these questions.
In 1966, twelve of the most heavily used insecticides (by weight of active ingredients only) were organochlorines. Farmers applied more than 75 million pounds of toxaphene, DDT, and aldrin, which were the three most popular insecticides. Most of the nearly 35 million pounds of toxaphene was used on cotton. Farmers also deployed nearly 30 million pounds of the five most popular cholinesterase inhibitors: a carbamate (carbaryl) and four organophosphate insecticides (ethyl parathion, methyl parathion, diazinon, and malathion). Some farmers used paraffinic oil as an alternative to other chemical insecticides. Oil killed insects by smothering them in place. As we have seen, ethyl parathion or parathion was one of the most toxic chemicals known, and the toxicity of methyl parathion is similar. Farmers applied more than 8 million pounds of both of these insecticides. The toxicity of the carbamate insecticide, carbaryl (aka Sevin), for humans and other animals, including birds, was very low, even nontoxic, but nontarget insects were highly susceptible. In using 12 million pounds of carbaryl, farmers may have shielded farm workers and wildlife. Diazinon had a classification of toxicity class II (moderately toxic) or toxicity class III (slightly toxic), but it was highly toxic to birds, fish, aquatic organisms, and nontarget insects, notably bees. Unlike most organophosphate insecticides, malathion posed slight toxicity to humans as well as to wildlife. Despite Rachel Carson’s warning in Silent Spring and the hearings following the recommendations of the PSAC, the use of pesticides rose to new levels in 1966. Chlorinated hydrocarbons appeared to swamp other insecticides, but the millions of pounds of highly toxic organophosphate insecticides posed significant risks to farm workers and wildlife.
By 1971,
with the wheels of regulation grinding slowly, organochlorine use had begun to fall. DDT use in agriculture, for example, had dropped to 14.3 million pounds, largely as a result of declining use in cotton growing, a decrease that was partially offset by increases in other insecticides.33 Similarly, aldrin use had fallen to less than 8 million pounds. Toxaphene use, however, had risen to a new high of 37.5 million pounds. Thus farmers used just under 60 million pounds of these insecticides, down from 75 million five years prior. Meanwhile, the use of organophosphate insecticides had climbed. Methyl parathion led this group with 27.5 million pounds (more than triple the amount applied in 1966). Use of the considerably less toxic carbaryl had also risen to nearly 18 million pounds, but use of ethyl parathion had climbed more than a million pounds to about 9.5 million pounds. With the application of more than 4 million pounds of phorate and disulfoton, two highly toxic organophosphate insecticides had entered agricultural insect control with a vengeance, but malathion use had dropped more than 1.5 million pounds to 3.6 million pounds, and diazinon use had also fallen (by more than 2 million pounds to 3.2 million pounds). Disulfoton stood out as a widely used systemic insecticide; when it was applied to soil, plant roots took up the insecticide and transferred it to all parts of the plant. It was particularly effective against sucking insects, such as aphids, leafhoppers, and thrips, while leaving predators and pollinators unharmed, for the most part. Having said that, disulfoton, which bound to soil, was highly toxic to aquatic organisms, fish, birds, and other wildlife.