Pesticides

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.

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