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Pesticides are chemicals or biological agents (such as viruses or bacteria) used to control or eliminate pests. Pests can be anything that disturbs human life, agriculture, horticulture, or domestic animals. Pests are most often insects, but they may be bacteria, fungi, weeds, unwanted fish, rodents, nematodes (roundworms), deer, or rabbits. Pests may be native to a local environment or invasive species. Pesticides are often classified according to the type of pest they are designed to control or eliminate. There are five kinds of pesticides: fungicides, herbicides, insecticides, rodenticides, and a miscellaneous category.
Fungicides are used against pathogenic fungi to either prevent the spread of the fungus or eliminate the infection. Fungi may infect plants, animals, and humans, damaging crops or human health. The damage may cost great sums of money, cause the loss of plants, damage the health of animals, or even kill humans. Contact fungicides are sprayed or used as an ointment on the infected area(s). Systemic fungicides kill fungi through an absorption mechanism that induces the fungi to absorb the fungicide, then sicken and die. Many cleaning products are fungicides. In some situations, a fungicide may be applied to garden or farm seed as a prophylactic. Usually, seed treated with a fungicide can be easily recognized because it is tinted a bright pink color to alert people to the danger of eating the seed. Fungicides (often containing mercury) have caused death, paralysis, or brain damage when people ate fungicide treated seed.
Herbicides are pesticides that are used to kill unwanted plants. There are two basic types of herbicides-general and specific. General herbicides, such as the commercial product Roundup, are nonspecific: All of the plants sprayed with a general herbicide are killed. Some general herbicides have been developed from plant hormones. They are applied to plants and are able to confuse hormonal growth patterns. Some plants are natural herbicides, such as walnuts (genus Juglans). Herbicides are used in enormous quantities in landscaping, landscape turf management, agriculture, and highway maintenance. Herbicides are also used extensively in the management of wildlife areas, in lakes, in forestry, and in pasture management systems. Specific herbicides kill only a specific type of plant; there are many commercial herbicides that can be applied to crab grass, poison ivy, or poison oak. Without these products, whole areas would have to be cleared by hand or with mechanical devices. There are some organic herbicides, which are usually expensive, confined to noncommercial uses, and less effective than synthetic chemical herbicides. Examples of organic herbicides include some spices, vinegar solutions, steam, and fire.
Insecticides are pesticides designed to eliminate crawling bugs, such as cockroaches, ants, fleas, flies, mites, and arachnids-ticks, mites, scorpions and spiders. Spiders and scorpions can be dangerous, and insecticides may be sprayed to make areas safe for human activities. Pyrethrums, which are obtained from chrysanthemums, and piperonyl butozide, derived from sassafras trees, are natural biological insecticides. Synthetic pyrethrums are currently finding wide use. Insecticides have improved human health and longevity by controlling insects that have plagued humans for millennia.
Rodenticides kill mice and rats. Those developed in the middle of the 20th century were usually composed of toxic substances such as arsenic, strychnine, cyanide, and even some chemicals developed for use as chemical weapons. Contemporary rodenticides are those that block the production of vitamin K. This compound plays an important role in coagulation. When rodenticides kill rodents, they may cause secondary poisoning in dogs or cats if they eat the weakened or dead rodent. Warfarrin and other rodenticides are designed to be toxic to rodents, but far less toxic to humans or larger animals.
Rodents carry fleas, which are carriers of numerous diseases. Bubonic plague and some hemorrhagic diseases are spread by fleas on rats and mice or by their droppings.
Among rodentborne diseases are arenavirus, hantavirus, leptospirosis, lymphocytic choriomeningitis, murine typhus, plague, rat bite fever, and salmonellosis. Cleansing human habitations and food stocks of pests is important to preserve human health.
The miscellaneous category of pesticides includes slug pellets (molluscicides), which kill snails, and various repellants for insects, birds, and animals. Rabbits in Australia have no natural predators. After their introduction, they multiplied enormously to the point where they were eating much of the existing vegetation. They were eventually controlled with a pesticide in the form of a virulent virus that is deadly to rabbits. Pesticide agents used in swimming pools remove health hazards, bacteria, and algae. A “trick” pesticide used to drive away birds or animals that become pests is to spray areas with the urine of their natural predators. Coyote urine has been used as nontoxic repellant; it is a biochemical pesticide, but also a naturally occurring substance that is otherwise harmless in nature. It has been used to repel armadillos, beavers, domestic cats, deer, elk, and wild pigs.
Without pesticides, crops would be lost and health improvements would be difficult. When crops are infested with a pest such as cabbage loopers, an entire crop can be destroyed or rendered unfit for use. Many localities use mass spraying of insecticides to control mosquitoes. The use of DDT (dichloro-diphenyl-trichloroethane) aided great reductions in typhus, malaria, and other insectborne diseases in World War II and afterward.
In 1962 Rachel Carson, an American biologist, published Silent Spring, which claimed that DDT was causing cancer and destroying bird populations. Her work was an important factor in the rise of the environmental movement and efforts to ban the use of insecticides. After the political controversy that arose following the publication of Silent Spring, Congress created the Environmental Protection Agency in 1971. It assumed control of pesticide regulation, taking over responsibility from the Department of Agriculture. One of its first actions was to ban DDT in 1972.
Environmentalists’ claims that chemical companies and other manufacturers of pesticides are producing dangerous products have been substantiated. Scientists, engineers, farmers and other agriculturalists also have a vested interest in not poisoning the land. Since the 1980s, pesticide research has sought out safer biological controls, such as pheromone traps and microbial agents. Modern pesticides used in urban areas have been found to pose little risk to human populations and are important defenses in the effort to control mosquito-borne diseases and destructive pests. Biological pesticides or those with reduced risks of side effects are being approved by the EPA in growing numbers.
- Rachel Carson, Silent Spring (Houghton Mifflin Company, 2002 );
- R. Coats and Hiroki Yamamoto, eds., Environmental Fate and Effects of Pesticides (American Chemical Society, 2003);
- Peter Hough, Global Politics of Pesticides: Forging Consensus from Conflicting Interests (Earthscan, 1998);
- Robert Krieger et , eds., Handbook of Pesticide Toxicology: Principles and Agents (Elsevier Science & Technology Books, 2001);
- B. Mandava, ed., Handbook of Natural Pesticides: Methods, Theory, Practice and Detection (CRC Press, 1985);
- Jorgen Stensersen and Steven Strauss, Chemical Pesticides: Modes of Action and Toxicology (CRC Press, 2004);
- William Vorley and Dennis Keeney, , Bugs in the System; Redesigning the Pesticide Industry for Sustainable Agriculture (Earthscan, 1998);
- F. Waxman, Agrochemical and Pesticides Handbook (CRC Press, 1998);
- David Weir and Mark Schapiro, Circle of Poison: Pesticides and People in a Hungry World (Food First, 1981).