Insecticides

The considerable benefits of many uses of insecticides are partially offset by damages caused to ecosystems and sometimes to human health. Each year about one million people are poisoned by pesticides (mostly by insecticides), including 20,000 fatalities. Although developing countries only account for about 20% of global pesticide use, they sustain about half of the poisonings. This is because highly toxic insecticides are used in many developing countries, but with poor enforcement of regulations, illiteracy, and inadequate use of protective equipment and clothing. The most spectacular case of pesticide-related poisoning occurred in 1984 at Bhopal, India. About 2,800 people were killed and 20,000 seriously poisoned when a factory accidentally released 44 tons (40 tonnes) of vapors of methyl isocyanate to the atmosphere. (Methyl isocyanate is a precursor chemical used to manufacture carbamate insecticides.)

In addition, many insecticide applications cause ecological damage by killing non-target organisms (that is, organisms that are not pests). These damages are particularly important when broad-spectrum insecticides (i.e., that are not toxic only to the pest) are sprayed over a large area, such as an agricultural field or a stand of forest. Broadcast sprays of this sort expose many non-target organisms to the insecticide and cause unintended but unavoidable mortality. For instance, broadcast insecticide spraying causes non-target mortality to numerous arthropods other than the pest species, and birds, mammals, and other creatures may also be poisoned. The non-target mortality may include predators and competitors of the pest species, which may cause secondary damage by releasing the pest from some of its ecological controls.

Some of the best-known damage caused by insecticides involves DDT and related organochlorines, such as DDD, dieldrin, aldrin, and others. These chemicals were once widely used in North America and other industrialized countries, but their use was banned in the early 1970s. DDT was first synthesized in 1874, and its insecticidal properties were discovered in 1939. The first successful uses of DDT were during the Second World War, in programs to control body lice, mosquitoes, and other vectors of human diseases. DDT was quickly recognized as an extremely effective insecticide, and immediately after the war it was widely used in agriculture, forestry, and spray programs against malaria. The manufacturing and use of DDT peaked in 1970, when 385 million lb (175 million kg) were produced globally. At about that time, however, developed countries began to ban most uses of DDT. This action was taken because of ecological damages that were being caused by its use, including the contamination of humans and their agricultural food web, and the possibility that this was causing human diseases. However, the use of DDT has continued in less-developed countries, especially in the tropics, and mostly in programs against mosquito vectors of diseases.

Two physical-chemical properties of DDT and other organochlorines have an important influence on their ecological damages: their persistence and high solubility in fats. Chlorinated hydrocarbons are highly persistent in the environment because they are not easily degraded by microorganisms or physical agents such as sunlight or heat. DDT has a typical half-life in soil of about three years. In addition, DDT and related organochlorines are extremely insoluble in water, so they cannot be "diluted" into this abundant solvent. However, these chemicals are highly soluble in fats or lipids (i.e., they are lipophilic), which mostly occur in organisms. Consequently, DDT and related organochlorines have a powerful affinity for organisms, and therefore bio-concentrate into organisms in strong preference to the non-living environment. Moreover, organisms are efficient at assimilating any organochlorines present in their food. As a result, predators at the top of the food web develop the highest residues of organochlorines, particularly in their fatty tissues (this is known as food-web magnification). Both bio-concentration and food-web magnification tend to be progressive with age, that is, the oldest individuals in a population are most contaminated. Although organochlorine residues are ubiquitous in the biosphere, much higher concentrations typically occur in animals that live close to areas where these chemicals have been used, such as North America.

Intense exposures to DDT and other organochlorines cause important ecological damages, including poisonings of birds. In some cases, bird kills were caused directly by the spraying of DDT in urban areas during the 1950s and 1960s to kill the beetle vectors of Dutch elm disease. So much bird mortality occurred in sprayed neighborhoods that there was a marked reduction of bird song—hence the title of Rachael Carson's (1962) book: Silent Spring, which is often considered a harbinger of the modern environmental movement in North America.

In addition to the direct toxicity of chlorinated hydrocarbons, more insidious damage was caused to birds and other wildlife over large regions. Mortality to many species was caused by longer-term, chronic toxicity, often occurring well away from sprayed areas. It took years of population monitoring and ecotoxicological research before organochlorines were identified as the causes of these damages. In fact, the chronic poisoning of birds and other wildlife can be considered an unanticipated "surprise" that occurred because scientists (and society) had not had experience with the longer-term effects of persistent, bio-accumulating organochlorines.

Species of raptorial birds were among the most prominent victims of organochlorine insecticides. These birds are vulnerable because they feed at the top of their food web, and therefore accumulate organochlorines to high concentrations. Breeding populations of various raptors suffered large declines. In North America these included the peregrine falcon (Falco peregrinus), osprey (Pandion haliaetus), bald eagle (Haliaeetus leucocephalus), and golden eagle (Aquila chrysaetos). In all cases, these birds were exposed to a "cocktail" of organochlorines that included the insecticides DDT, DDD (both of which are metabolized to DDE in organisms), aldrin, dieldrin, and heptachlor, as well as PCBs, a non-insecticide with many industrial uses. Research has suggested that DDT was the more important toxin to birds in North America, while cyclodienes (particularly dieldrin) were more important in Britain.

Damage caused to predatory birds was largely associated with chronic effects on reproduction, rather than toxicity to adults. Reproductive damages included the production of thin eggshells that could break under the weight of an incubating parent, high death rates of embryos and nestlings, and abnormal adult behavior. These effects all contributed to decreases in the numbers of chicks raised, which resulted in rapid declines in the sizes of populations of the affected birds.

Since the banning of most uses of DDT and other organochlorines in North America, their residues in wildlife have been declining. This has allowed previously affected species to increase in abundance. In 1999, for example, the U.S. Fish and Wildlife Service removed the peregrine falcon from the list of species considered endangered. Although the population recovery of the peregrine falcon was aided by a program of captive-breeding and release, its recovery would not have been possible if their exposure to organochlorines in wild habitats had not been first dealt with.

DDT and related organochlorine insecticides have largely been replaced by organophosphate and carbamate chemicals. These chemicals poison insects and other arthropods by inhibiting a specific enzyme, acetylcholine esterase (AChE), which is critical in the transmission of neural impulses. Vertebrates such as amphibians, fish, birds, and mammals are also highly sensitive to poisoning of their cholinesterase enzyme system. In all of these animals, acute poisoning of the AChE function by organophosphate and carbamate insecticides can cause tremors, convulsions, and ultimately death to occur.

Carbofuran is a carbamate insecticide that caused much bird mortality during its routine agricultural usage. For this reason, the further use of this chemical was banned in North America during the late 1990s. In 1996, it was discovered that agricultural use of the organophosphate monocrotophos against grasshoppers in Argentina was killing large numbers of Swainson's hawks (Buteo swainsoni). This raptor breeds in the western United States and Canada and winters on the pampas of South America. Populations of Swain-son's hawks had been declining for about 10 years, and it appears the cause was poorly regulated use of monocrotophos on their wintering grounds. Because of risks of ecological damages caused by its use, monocrotophos has been banned in the United States and was never registered for use in Canada, but it could be legally used in Argentina. These are two examples of non-organochlorine insecticides that cause important ecological damages.

Of course, not all insecticides cause these kinds of serious ecological damages. For example, the toxicity of the bacterial insecticide B.t. is largely limited to moths, butterflies, beetles, and flies—its is essentially non-toxic to most other invertebrates or vertebrate animals. Other relatively pest-specific insecticides are being developed and are increasing rapidly in use, often in conjunction with a so-called "integrated pest management" (or IPM) system. In IPM, insecticides may be used as a method of last resort, but heavy reliance is also placed on other methods of pest management. These include the cultivation of pest-resistant crop varieties, growing crops in rotation, modifying the habitat to make it less vulnerable to infestation, and other practices that reduce the overall impacts of pest insects.

The continued development of pest-specific insecticides and IPM systems will further reduce society's reliance on broad-spectrum insecticides and other damaging pesticides. Until this happens, however, the use of relatively damaging, broad-spectrum insecticides will continue in North America. In fact, the use of these chemicals is rapidly increasing globally, because they are becoming more prevalent in less-developed countries of tropical regions.