DDT (Dichlorodiphenyl-Trichloroacetic Acid)

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 DDT sperulites magnified 50 times. Photograph by David Malin. Photo Researchers, Inc. Reproduced by permission. 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.