Agrochemicals

Pesticides are agrochemicals that are used to reduce the abundance of pests, that is, organisms that are considered to interfere with some human purpose. Many kinds of pesticides are used in agriculture, but they can be categorized into simple groups on the basis of the sorts of pests that are the targets of the use of these chemicals. Herbicides are used to kill weeds, that is, non-desired plants that interfere with the growth of crops and thereby reduce their yield. Fungicides are used to protect agricultural plants from fungal pathogens, which can sometimes cause complete failure of crops. Insecticides are used to kill insects that defoliate crops, or that feed on stored grains or other agricultural products. Acaricides (or miticides) are used to kill mites, which are important pests of crops such as apples, and ticks, which can carry debilitating diseases of livestock. Nematicides are used to kill nematodes, which are important parasites of the roots of some crop species. Rodenticides are used to kill rats, mice, gophers, and other rodents that are pests in fields or that eat stored crops. Preservatives are agrochemicals added to processed foods to help prevent spoilage.

Pesticides are chemically diverse substances. About 300 different insecticides are now in use, along with about 290 herbicides, 165 fungicides, and other pesticides. However, each specific pesticidal chemical (also known as the "active ingredient") may be marketed in a variety of formulations, which contain additional substances that act to increase the efficacy of the actual pesticide. These so-called "inert" ingredients of the formulation can include solvents, detergents, emulsifiers, and chemicals that allow the active ingredient to adhere better to foliage. In total, more than 3,000 different pesticidal formulations exist.

Pesticides can also be classified according to the similarities of their chemical structures. Inorganic pesticides, for example, are simple compounds of toxic elements such as arsenic, copper, lead, and mercury. Inorganic pesticides were formerly used in large quantities, especially as fungicides. However, they have largely been replaced by various organic (carbon-containing) pesticides.

A few of the commonly used organic pesticides are based on substances that are synthesized naturally by plants as biochemical defenses, and can be extracted and used against pests. Pyrethrin, for example, is an insecticide based on pyrethrum, which is obtained from a species of chrysanthemum, while rotenone is a rodenticide extracted from a tropical shrub.

Most organic pesticides, however, have been synthesized by chemists. The synthetic organic pesticides include such well-known groups as the chlorinated hydrocarbons (including the insecticide DDT, and the herbicides 2,4-D and 2,4,5-T), organophosphates (such as parathion and malathion), carbamates (for example, carbaryl and carbofuran), and triazine herbicides (such as atrazine and simazine).

A final class of pesticides is based on the action of bacteria, fungi, or viruses that are pathogenic to specific pests and can be applied as a pesticidal formulation. The most commonly used biological insecticide is manufactured using spores of the bacterium Bacillus thuringiensis, also known as Bt. These spores can be mass-produced in laboratory-like factories, and then used to prepare an insecticidal solution. Insecticides containing Bt are mostly used against leaf-eating moths, biting flies, such as black-flies, and mosquitoes. Most other insects are little affected by Bt-based insecticides, so the unintended nontarget effects of their usage are relatively small.

Extremely large quantities of pesticides are used in modern agriculture. Globally, about 4.4-6.6 billion lb (2-3 billion kg) of pesticides are used each year, having a total value of about $20 billion. The United States alone accounts for about one-third of all pesticide usage, even though that country only supports about 4% of the world's population.