DDT (Dichlorodiphenyl-trichloroacetic acid)

Dichlorodiphenyl-trichloroacetic acid (or DDT) is a chlorinated hydrocarbon that has been widely used as an insecticide. DDT is virtually insoluble in water , but is freely soluble in oils and in the fat of organisms. DDT is also persistent in the environment. The combination of persistence and lipid solubility means that DDT biomagnifies, occurring in organisms in preference to the nonliving environment, especially in predators at the top of ecological food webs. Environmental contamination by DDT and related chemicals is a widespread problem, including the occurrence of residues in wildlife , in drinking water, and in humans. Ecological damage has included the poisoning of wildlife, especially avian predators.

DDT and other chlorinated hydrocarbons

Chlorinated hydrocarbons are a diverse group of synthetic compounds of carbon , hydrogen , and chlorine , used as pesticides and for other purposes. DDT is a particular chlorinated hydrocarbon with the formula 2,2-bis-(p-chlorophenyl)-1,1,1-trichloroethane.

The insecticidal relatives of DDT include DDD, aldrin, dieldrin, heptachlor, and methoxychlor. DDE is a related non-insecticidal chemical, and an important, persistent, metabolic-breakdown product of DDT and DDD that accumulates in organisms. Residues of DDT and its relatives are persistent in the environment, for example, having a typical half-life of 5-10 years in soil .

A global contamination with DDT and related chlorinated hydrocarbons has resulted from the combination of their persistence and a tendency to become widely dispersed with wind-blown dusts. In addition, their selective partitioning into fats and lipids causes these chemicals to bioaccumulate. Persistence, coupled with bioaccumulation , results in the largest concentrations of these chemicals occurring in predators near or at the top of ecological food webs.

Uses of DDT

DDT was first synthesized in 1874. Its insecticidal qualities were discovered in 1939 by Paul Muller, a Swiss scientist who won a Nobel Prize in medicine in 1948 for his research on the uses of DDT. The first important use of DDT was for the control of insect vectors of human diseases during and following World War II. At about that time the use of DDT to control insect pests in agriculture and forestry also began.

The peak production of DDT was in 1970 when 386 million lb (175 million kg) were manufactured globally. The greatest use of DDT in the United States was 79 million lb (36 million kg) in 1959, but the maximum production was 199 million lb (90 million kg) in 1964, most of which was exported. Because of the discovery of a widespread environmental contamination with DDT and its breakdown products, and associated ecological damage, most industrialized countries banned its use in the early 1970s. The use of DDT continued elsewhere, however, mostly for control of insect vectors of human and livestock diseases in less-developed, tropical countries. Largely because of the evolution of resistance to DDT by many pest insects , its effectiveness for these purposes has decreased. Some previously well-controlled diseases such as malaria have even become more common recently in a number of countries (the reduced effectiveness of some of the prophylactic pharmaceuticals used to threat malaria is also importance in the resurgence of this disease ). Eventually, the remaining uses of DDT will probably be curtailed and it will be replaced by other insecticides , largely because of its increasing ineffectiveness.

Until its use was widely discontinued because of its non-target, ecological damages, DDT was widely used to kill insect pests of crops in agriculture and forestry and to control some human diseases that have specific insect vectors. The use of DDT for most of these pest-control purposes was generally effective. To give an indication of the effectiveness of DDT in killing insect pests, it will be sufficient to briefly describe its use to reduce the incidence of some diseases of humans.

In various parts of the world, species of insects and ticks are crucial as vectors in the transmission of diseasecausing pathogens of humans, livestock, and wild animals. Malaria, for example, is a debilitating disease caused by the protozoan Plasmodium and spread to people by mosquitoes , especially species of Anopheles. Yellow fever and related viral diseases such as encephalitis are spread by other species of mosquitoes. The incidence of these and some other important diseases can be greatly reduced by the use of insecticides to reduce the abundance of their arthropod vectors. In the case of mosquitoes, this can be accomplished by applying DDT or another suitable insecticide to the aquatic breeding habitat , or by applying a persistent insecticide to walls and ceilings which serve as resting places for these insects. In other cases, infestations of body parasites such as the human louse can be treated by dusting the skin with DDT.

The use of DDT has been especially important in reducing the incidence of malaria, which has always been an important disease in warmer areas of the world. Malaria is a remarkably widespread disease, affecting more than 5% of the world's population each year during the 1950s. For example, in the mid-1930s an epidemic in Sri Lanka affected one-half of the population, and 80,000 people died as a result. In Africa , an estimated 2-5 million children died of malaria each year during the early 1960s.

The use of DDT and some other insecticides resulted in large decreases in the incidence of malaria by greatly reducing the abundance of the mosquito vectors. India, for example, had about 100 million cases of malaria per year and 0.75 million deaths between 1933 and 1935. In 1966, however, this was reduced to only 0.15 million cases and 1,500 deaths, mostly through the use of DDT. Similarly, Sri Lanka had 2.9 million cases of malaria in 1934 and 2.8 million in 1946, but because of the effective use of DDT and other insecticides there were only 17 cases in 1963. During a vigorous campaign to control malaria in the tropics in 1962, about 130 million lb (59 million kg) of DDT was used, as were 7.9 million lb (3.6 million kg) of dieldrin and one million lb (0.45 million kg) of lindane. These insecticides were mostly sprayed inside of homes and on other resting habitat of mosquitoes, rather than in their aquatic breeding habitat. More recently, however, malaria has resurged in some tropical countries, largely because of the development of insecticide resistance by mosquitoes and a decreasing effectiveness of the pharmaceuticals used to prevent the actual disease.

Environmental effects of the use of DDT

As is the case with many actions of environmental management, there have been both benefits and costs associated with the use of DDT. Moreover, depending on socio-economic and ecological perspectives, there are large differences in the perceptions by people of these benefits and costs. The controversy over the use of DDT and other insecticides can be illustrated by quoting two famous persons. After the successful use of DDT to prevent a potentially deadly plague of typhus among Allied troops in Naples during World War II, Winston Churchill praised the chemical as "that miraculous DDT powder." In stark contrast, Rachael Carson referred to DDT as the "elixir of death" in her ground-breaking book Silent Spring, which was the first public chronicle of the ecological damage caused by the use of persistent insecticides, especially DDT.

DDT was the first insecticide to which large numbers of insect pests developed genetically based resistance. This happened through an evolutionary process involving selection for resistant individuals within large populations of pest organisms exposed to the toxic pesticide. Resistant individuals are rare in unsprayed populations, but after spraying they become dominant because the insecticide does not kill them and they survive to reproduce and pass along their genetically based tolerance. More than 450 insects and mites have populations that are resistant to at least one insecticide. Resistance is most common in the flies (Diptera), with more than 155 resistant species, including 51 resistant species of malaria-carrying mosquito, 34 of which are resistant to DDT.

As mentioned previously, the ecological effects of DDT are profoundly influenced by certain of its physical/chemical properties. First, DDT is persistent in the environment because it is not readily degraded to other chemicals by microorganisms , sunlight, or heat . Moreover, DDE is the primary breakdown product of DDT, being produced by enzymatic metabolism in organisms or by inorganic de-chlorination reactions in alkaline environments. The persistences of DDE and DDT are similar, and once released into the environment these chemicals are present for many years.

Another important characteristic of DDT is its insolubility in water, which means that it cannot be "diluted" into this ubiquitous solvent, so abundant in Earth's environments and in organisms. In contrast, DDT is highly soluble in fats (or lipids) and oils, a characteristic shared with other chlorinated hydrocarbons. In ecosystems, most lipids occur in the tissues of living organisms. Therefore, DDT has a strong affinity for organisms because of its high lipid solubility, and it tends to biomagnify tremendously. Furthermore, top predators have especially large concentrations of DDT in their fat, a phenomenon known as food-web accumulation. In ecosystems, DDT and related chlorinated hydrocarbons occur in extremely small concentrations in water and air. Concentrations in soil may be larger because of the presence of organic matter containing some lipids. Larger concentrations occur in organisms, but the residues in plants are smaller than in herbivores, and the highest concentrations occur in predators at the top of the food web, such as humans, predatory birds , and marine mammals . For example, DDT residues were studied in an estuary on Long Island where DDT had been sprayed onto salt marshes to kill mosquitoes. The largest concentrations of DDT occurred in fish-eating birds such as ring-billed gull (76 ppm), and double-crested cormorant, red-breasted merganser, and herring gull (range of 19-26 ppm).

Lake Kariba, Zimbabwe, is a tropical example of food-web bioconcentration of DDT. Although Zimbabwe banned DDT use in agriculture in 1982, it is still used to control mosquitoes and tsetse fly (a vector of diseases of cattle and other large mammals). The concentration of DDT in water of Lake Kariba was extremely small, less than 0.002 ppb, but larger in sediment of the lake (0.4 ppm). Algae contained 2.5 ppm, and a filter-feeding mussel contained 10 ppm in its lipids. Herbivorous fish contained 2 ppm, while a bottom-feeding species of fish contained 6 ppm. The tigerfish and cormorant (a bird) feed on small fish, and these contained 5 ppm and 10 ppm, respectively. The top predator in Lake Kariba is the Nile crocodile, and it contained 34 ppm. Lake Kariba exhibits a typical pattern for DDT and related chlorinated hydrocarbons; a large bio-concentration from water, and to a lesser degree from sediment, as well as a food-web magnification from herbivores to top predators.

Global contamination with DDT

Another environmental feature of DDT is its ubiquitous distribution in at least trace concentrations everywhere in the biosphere . This global contamination with DDT, and related chlorinated hydrocarbons such as PCBs, occurs because they enter into the atmospheric cycle and thereby become very widely distributed. This results from: (1) a slow evaporation of DDT from sprayed surfaces; (2) off-target drift of DDT when it is sprayed; and (3) entrainment by strong winds of DDT-contaminated dust into the atmosphere.

This ubiquitous contamination can be illustrated by the concentrations of DDT in animals in Antarctica , very far from places where it has been used. DDT concentrations of 5 ppm occur in fat of the southern polar skua, compared with less than 1 ppm in birds lower in the food web of the Southern Ocean such as the southern fulmar and species of penguin.

Much larger concentrations of DDT and other chlorinated hydrocarbons occur in predators living closer to places where the chemicals have been manufactured and used. The concentration of DDT in seals off the California coast was as much as 158 ppm in fat during the late 1960s. In the Baltic Sea of Europe residues in seals were up to 150 ppm, and off eastern Canada as much as 35 ppm occurred in seals and up to 520 ppm in porpoises.

Large residues of DDT also occur in predatory birds. Concentrations as high as 356 ppm (average of 12 ppm) occurred in bald eagles from the United States, up to 460 ppm in western grebes , and 131 ppm in herring gulls . White-tailed eagles in the Baltic Sea have had enormous residues—as much as 36,000 ppm of DDT and 17,000 ppm PCBs in fat, and eggs with up to 1,900 ppm DDT and 2,600 ppm PCBs.

Ecological damage

Some poisonings of wildlife were directly caused by exposure to sprays of DDT. There were numerous cases of dying or dead birds being found after the spraying of DDT, for example, after its use in residential areas to kill the beetle vectors of Dutch elm disease in North America . Spray rates for this purpose were large, about 1.5-3.0 lb (1.0-1.5 kg) of DDT per tree , and resulted in residues in earthworms of 33-164 ppm. Birds that fed on DDT-lacedinvertebrates had intense exposures to DDT, and many were killed.

Sometimes, detailed investigations were needed to link declines of bird populations to the use of organochlorines. One such example occurred at Clear Lake, California, an important waterbody for recreation. Because of complaints about the nuisance of a great abundance of non-biting aquatic insects called midges, Clear Lake was treated in 1949 with DDD at 1 kg/ha. Prior research had shown that this dose of DDD would achieve control of the midges but would have no immediate effect on fish. Unfortunately, the unexpected happened. After another application of DDD in 1954, 100 western grebes were found dead as were many intoxicated birds. Eventually, the breeding population of these birds on Clear Lake decreased from about 2,000 to none by 1960. The catastrophic decline of grebes was linked to DDD when an analysis of the fat of dead birds found residues as large as 1,600 ppm. Fish were also heavily contaminated. The deaths of birds on Clear Lake was one of the first well documented examples of a substantial mortality of wildlife caused by organochlorine insecticides.

Damage to birds also occurred in places remote from sprayed areas. This was especially true of raptorial (that is, predatory) birds, such as falcons , eagles, and owls . These are top predators, and they food-web accumulate chlorinated hydrocarbons to high concentrations. Declines of some species began in the early 1950s, and there were extirpations of some breeding populations. Prominent examples of predatory birds that suffered population declines from exposure to DDT and other organochlorines include the bald eagle, golden eagle, peregrine falcon , prairie falcon , osprey, brown pelican, double-crested cormorant, and European sparrowhawk.

Of course, birds and other wildlife were not only exposed to DDT. Depending on circumstances, there could also be significant exposures to other chlorinated hydrocarbons, including DDD, aldrin, dieldrin, heptachlor, and PCBs. Scientists have investigated the relative importance of these chemicals in causing the declines of predatory birds. In Britain, the declines of raptors did not occur until dieldrin came into common use, and this insecticide may have been the primary cause of the damage. However, in North America DDT use was more common, and it was probably the most important cause of the bird declines there.

The damage to birds was mainly caused by the effects of chlorinated hydrocarbons on reproduction, and not by direct toxicity to adults. Demonstrated effects of these chemicals on reproduction include: (1) a decrease in clutch size (i.e., the number of eggs laid); (2) the production of a thin eggshell which might break under the incubating parent; (3) deaths of embryos, unhatched chicks, and nestlings; and (4) pathological parental behavior . All of these effects could decrease the numbers of young successfully raised. The reproductive pathology of chlorinated hydrocarbons caused bird populations to decrease because of inadequate recruitment.

This syndrome can be illustrated by the circumstances of the peregrine falcon, a charismatic predator whose decline attracted much attention and concern. Decreased reproductive success and declining populations of peregrines were first noticed in the early 1950s. In 1970, a North American census reported almost no successful reproduction by the eastern population of peregrines, while the arctic population was declining in abundance. Only a local population in the Queen Charlotte Islands of western Canada had normal breeding success and a stable population. This latter population is non-migratory, inhabiting a region where pesticides are not used and feeding largely on non-migratory seabirds. In contrast, the eastern peregrines bred where chlorinated hydrocarbon pesticides were widely used, and its prey was generally contaminated. Although the arctic peregrines breed in a region where pesticides are not used, these birds winter in sprayed areas in Central and South America where their food is contaminated, and their prey of migratory ducks on the breeding grounds is also contaminated. Large residues of DDT and other organochlorines were common in peregrine falcons (except for the Queen Charlottes). Associated with those residues were eggshells thinner than the pre-DDT condition by 15-20% and a generally impaired reproductive rate .

In 1975, another North American survey found a virtual extirpation of the eastern peregrines, while the arctic population had declined further and was clearly in trouble. By 1985 there were only 450 pairs of arctic peregrines, compared with the former abundance of 5,000-8,000. However, as with other raptors that suffered from the effects of chlorinated hydrocarbons, a recovery of peregrine populations has begun since DDT use was banned in North America and most of Europe in the early 1970s. In 1985, arctic populations were stable or increasing compared with 1975, as were some southern populations, although they remained small. This recovery has been enhanced by a captive-breeding and release program over much of the former range of the eastern population of peregrine falcons.

It is still too soon to tell for certain, but there are encouraging signs that many of the severe effects of DDT and other chlorinated hydrocarbons on wildlife are becoming less severe. Hopefully, in the future these toxic damages will not be important.

See also Biomagnification.

Resources

books

Freedman, B. Environmental Ecology. 2nd ed. San Diego: Academic Press, 1994.

Smith, R.P. A Primer of Environmental Toxicology. Philadelphia: Lea & Febiger, 1992.

Bill Freedman

KEY TERMS

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Drift

—Movement of sprayed pesticide by wind beyond the intended place of treatment.

Ecotoxicology

—The study of the effects of toxic chemicals on organisms and ecosystems. Ecotoxicology considers both direct effects of toxic substances and also the indirect effects caused, for example, by changes in habitat structure or the abundance of food.

Vector

—Any agent, living or otherwise, that carries and transmits parasites and diseases.