4.2 DDT: a classic case in ecotoxicology
DDT is very effective in controlling pests, being very toxic to insects, and is cheap to produce. Its effectiveness is enhanced because it is very persistent, remaining active in the environment for a long time. This increases its value as an insecticide to farmers because one application lasts a long time, but is also a major reason why it poses a threat to wildlife and to human health. Although the agricultural use of DDT was banned in most developed countries 30 years ago, it can still be detected in samples taken from the soil, from water and from the bodies of animals; it is also detectable in people.
Concern about the widespread effects of DDT led to it being banned in most developed countries in the 1960s and 1970s, but it is still used extensively in many developing countries, not least because it is very effective at controlling disease-carrying insects. DDT is credited with having eliminated malaria from Europe and the USA. The WHO has recently agreed that the human health benefits of using DDT in malaria areas outweigh the environmental risks, and it is a front-line weapon against the mosquitoes that carry malaria. Efforts are being made, however, to reduce the quantities used; for example, DDT-impregnated bed nets are increasingly been used instead of spraying (Figure 9).
The harmful effects of DDT on wildlife were first detected in 1947 when dramatic declines in a number of birds of prey, particularly the peregrine falcon, were noted in Britain. Similar declines were detected in the USA, especially among birds, such as ospreys, that eat fish. By the early 1960s the link between such declines and DDT had been worked out.
Why are birds like ospreys and peregrine falcons at particular risk?
Peregrines and ospreys prey on mammals and fish and so occupy a position at the top of food chains, in which DDT-affected insects occupy a low level, so that, through bioaccumulation, they build up DDT in high concentrations in their body fat.
The most severely poisoned birds died; those that did not die produced eggs with shells so thin that they collapsed under the weight of the parents in the nest. The use of the breeding birds’ fat reserves during egg production released large amounts of DDT, affecting the deposition of calcium in egg shells. This is an example of a phenomenon called endocrine disruption (see Section 4.5).
DDT has been suggested as a causal factor for a number of human health problems. Detailed studies have ruled out a suggested link between DDT and breast cancer, but it is still strongly suspected to be linked to pancreatic cancer, neuropsychological dysfunction and some reproductive problems (Beard, 2006).
DDT is only one of a vast array of pesticides that is now in use. In 1995, 2.6 million metric tons of active pesticide ingredients, worth $38 billion, were used around the world (WRI, 1999). 85% of this was used in agriculture, much of which produces crops in developing countries, such as cotton, bananas, coffee, vegetables and flowers, that are exported to developed countries. In 2000, an estimated 3.75 million metric tons were manufactured worldwide; on current trends, global pesticide production is predicted to reach 6.55 million metric tons in 2025 and 10.1 million in 2050 (Tilman et al., 2001).