Insecticides | Research & Encyclopedia Articles

Insecticides

Insecticides are chemical substances used to destroy or control insect populations that could otherwise do billions of dollars in damage annually. Although the primary use of insecticides is in agriculture and animal husbandry, insecticides also help to control the spread of insect-borne diseases and to protect buildings, households, and products such as clothing and books. Historically, many kinds of insecticides have been developed, as no single poison is effective against all insect pests--indeed, most insecticides do not even remain effective against a single pest indefinitely. Insecticides can be broadly classified, based on their chemical structure, as inorganic or organic. The main ingredient of inorganic insecticides is of mineral origin, such as arsenic. Organic insecticides, whether derived from plants or made synthetically, consist of carbon in combination with one or more elements such as hydrogen, oxygen, sulfur, nitrogen, phosphorus, chlorine, or bromine. The toxicity of inorganic insecticides depends on the presence of certain key elements; the toxicity of organic insecticides is determined by their molecular structure. Insecticide poisons can also be classified in terms of their action. Some poisons penetrate an insect's exterior membranes, whether applied directly ( contact insecticides) or to surfaces that the insects will touch (residual insecticides). Stomach poisons rely on ingestion, while fumigants enter the insect through its respiratory system. Modern insecticides often combine these methods, and are applied in a variety of formulations, including dusts and sprays. Insect control dates back two thousand years to the ancient Chinese, who used pyrethrum, extracted from the chrysanthemum flower, to kill fleas and lice. The Chinese also used arsenic sulfide to deter various plant-eating insects. In classical times, the Greeks and Romans applied sulfur and other insect-deterring substances to their crops. The Europeans discovered arsenic in the Middle Ages, and by 1681 arsenicals were being used as insecticides. In 1690, French agronomist Jean de la Quintinie discovered that tobacco was also highly effective as an insecticide. The French subsequently used tobacco against plant lice, although its active ingredient was only later identified as nicotine. By the nineteenth century, the need for insecticides had intensified, due to the increasingly large-scale cultivation of single crops and to the exportation of insect species, on cargo ships and passenger steamers, far beyond their native habitats. Among the most important insecticides developed in the nineteenth century was Paris green (copper acetoarsenite), discovered around 1868. Paris green soon became the best-known arsenic insecticide and the first chemical insecticide to be applied on a large scale. In addition to helping combat a Colorado potato beetle infestation of the American midwest in 1872, Paris green led to the development of London purple and lead arsenate, which proved effective against the potentially devastating gypsy moth. Sulfur was also used to fight red spiders and plant lice. Though widespread application of natural organic and inorganic insecticides won some battles in the nineteenth century, insects were still winning the war. Thus, the American journalist Horace Greely reported in 1879 that the average annual loss to farmers from insect damage exceeded $100 million.

Certain insects, moreover, such as the boll weevil that destroyed cotton crops in Texas, were immune to the natural insecticides available at that time. Only after years of testing was the spread of the boll weevil checked by a new pesticide, calcium arsenate. Just prior to World War II, when organic insecticides were first created synthetically in the laboratory, scientists thought they had finally discovered the ultimate tool to prevent crop destruction. The first synthetic insecticide to be developed was of the chlorinated hydrocarbon type. In 1939, the insecticidal properties of dichloro-diphenyl-trichloroethane, known as DDT, were discovered by the Swiss chemist Paul Hermann Müller (1899-1965), who was awarded the Nobel Prize in 1948 for this accomplishment. This colorless and odorless pesticide was found extremely effective against both insect pests and crop-threatening insects. Furthermore, DDT was used successfully during World War II to protect American troops in the Pacific from malaria-carrying mosquitoes, and then to all but eradicate malaria domestically within the United States. DDT remains the most familiar of chlorinated hydrocarbon insecticides not only because of such benefits, however, but because of the subsequent realization of its capacity for large-scale ecological damage. DDT's extreme durability results in its transference into living organisms that are not intended targets, such as birds and fish. The ingestion of DDT, for example, has come close to exterminating the American falcon. Humans themselves are at risk when high concentrations of DDT accumulate within the body. DDT ultimately pollutes the entire food chain. In 1972, therefore, less than thirty years after its discovery, the United States Environmental Protection Agency banned the agricultural use of DDT. Other chlorinated hydrocarbon insecticides, such as chlordane, are similarly proscribed both domestically and abroad. The destructive capabilities of the second major group of synthetic insecticides, organophosphoric compounds, became apparent even sooner in their history. Discovered by the German chemist Gerhardt Schrader in the late 1930s, these compounds affect living organisms by targeting the nervous system. Whether the victim is an insect or a warm-blooded animal, they cause violent muscular convulsions before an almost immediate death. The German government, recognizing the potential of these phosphorous compounds for military application, developed them for use during World War II, but they were not deployed. The development of organophosphoric compounds as insecticides was spurred after the war by Schrader's subsequent discovery that they could penetrate and spread through plant tissues without losing their insecticidal effect. Thousands of organic phosphorous insecticides, including parathion and malathion, were then synthesized for agricultural purposes before public opposition and government restriction gradually abated their use. The increasing rejection of highly toxic insecticides, whether inorganic or organic, has led to a search for safer and more humane solutions. Insect repellents or attractants can thus be used (the latter lead insects away from crops and towards other food sources). Several sophisticated methods of impeding insect growth or reproduction have also been developed, and insect populations are being controlled through natural predators, through the introduction of sterile males to attract females, or through formulations that can be ingested by humans and animals but that damage insects at the larval stage. Human attemptsat intervention in ecological systems have led to many disasters, however, regardless of how apparently humane or "natural" the intervention. Finding new ways to achieve food production and disease control without unleashing the potential ecological destructiveness of insecticides remains an important public, commercial, and governmental priority.