DDT

DDT

DDT (dichloro-diphenyl-trichloro-ethane) is an insecticide that was first used worldwide in 1946 to increase agricultural production and to reduce disease vectors (carriers). Although formulated in 1874, DDT's insecticidal properties were not discovered until 1936. Paul Muller of Switzerland won the Nobel Prize for that discovery in 1948.

The neurotoxin DDT interferes with the action potential along neurons. It affects insects and vertebrates by means of this same primary mechanism. It has a greater effect on insects simply because they are smaller and absorb it more readily. At a high enough dosage, DDT can have as detrimental an effect on vertebrates, including humans. Symptoms of DDT toxicity include apprehension, headache, anorexia, nausea, hyper-excitability, muscle fibrillation, respiratory arrest, coma, and death.

DDT is relatively inert and stable, and is nearly insoluble in water. This combination of attributes allows it to be stored easily in fat. As a result, fatty tissues act as biological magnifiers by slowing the excretion of DDT after it is absorbed. Like other fat-soluble compounds, DDT is transferred up the food chain more efficiently than are water-soluble compounds, thereby achieving higher concentrations among carnivores. Because DDT toxicity is a function of concentration, this biomagnification is most likely to cause problems for a predator species, such as eagles, ospreys, and falcons, at the end of a long food chain.

During the 1950s, United States efforts to control Dutch elm disease consisted primarily of killing its vector, elm bark beetles, with DDT. Soon after, communities in the Midwest and Northeast began to notice an accumulation of dead robins and other birds. Roy Barker discovered that earthworms were consuming DDT sprayed on the elm trees which was seeping into the soil. Robins in turn ate the earthworms, receiving a lethal dose of DDT from as few as eleven worms. Birds that did not die often suffered reduced fertility.

Other predatory bird species also appeared to decline during the 1940s and 1950s. Hawk Mountain, Pennsylvania, is a stopover for many migrating hawks and eagles. From 1935 to 1939, 40 percent of the eagles were yearlings. Between 1955 and 1959, only 20 percent were yearlings. The percentage of juveniles in eagle populations along the Mississippi, Illinois, and Susquehanna Rivers also declined after 1947. In 1950, there were two hundred mating pairs of ospreys at the mouth of the Connecticut River; by 1970 the number had dropped to six. One study suggested that DDT interfered with calcium deposition in eggshells, thereby potentially reducing the reproduction rate of susceptible bird species. However, subsequent studies found no correlation between DDT levels and eggshell thickness either in nature or in controlled experiments.

DDT may adversely affect whole ecosystems. In the 1950s, the Canadian government instituted a policy of eradicating the spruce budworm. This native insect attacks several species of evergreens. Millions of acres of the Northwest Miramichi watershed were sprayed to save balsams, the pulp industry's most valuable cash crop. Soon trout and salmon began to turn up dead along the streams. Their prey, caddis fly larvae, stonefly nymphs, and blackfly larvae, were being killed along with the spruce budworm. In 1959, the watershed produced less than a third of the smolt (young salmon) it had produced before the most recent spraying.

Rachel Carson's book Silent Spring, was released in 1962 and documented the adverse effects of DDT on the environment. Her book catalyzed the modern American environmental movement. Over the years a variety of objections have been made to her characterization of DDT as an absolute detriment to human and ecological health. For example, studies on the role of DDT in breast cancer have yielded ambiguous results. It is also possible that the correlation between DDT use and the decline of fish and bird populations was caused by the simultaneous use of other pollutants such as PCBs.

The Environmental Protection Agency banned DDT in 1971 as a potential human carcinogen. Since then most countries have banned the chemical for agricultural purposes. However, DDT continues to be a cheap and effective way to kill mosquitoes that transmit malaria, requiring lower concentrations than those for agricultural use.

In 2000, more than 100 governmental and nongovernmental agencies gathered to formulate a treaty to completely phase out DDT and eleven other pollutants. The World Health Organization warned that a sudden worldwide ban on DDT could result in an epidemic of malaria in countries that cannot afford other effective insecticides. Until safe, affordable alternatives are developed, DDT will continue to be used in many countries where malaria is endemic, and its residues will be found in soils and human breast milk around the world. span>

see also Carson, Rachel; Pesticides; Silent Spring.

Brian R. West

Bibliography

Carson, Rachel. Silent Spring. Boston, MA: Houghton Mifflin, 1962.

DDT and Its Derivatives: Environmental Aspects. Geneva, Switzerland: World Health Organization, 1989.

Internet Resources

Note for the Press No. 15. World Health Organization. <http://www.who.org>.