DDT (Dichlorodiphenyl-Trichloroacetic Acid)

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.