Pesticide Use And Classification

A wide variety of the types of pesticides is used in food production. Modern pesticides include synthetically produced organic chemicals, naturally occurring organic and inorganic chemicals, and microbial agents (natural or obtained through genetic manipulation). Table 1 lists many types of pesticides and their targets. Weeds, insects, and fungi are the major pests responsible for damage to agriculture. In terms of pounds applied, herbicides accounted for 55% of U.S. pesticide use in 1995, followed by

Table 1. Pesticide Types and Targets

Pesticide type

Pest controlled

Insecticide

Insect

Herbicide

Weeds

Fungicide

Fungi

Nematicide

Nematodes

Acaricide

Mites

Defoliant

Leaves

Bactericide

Bacteria

Rodenticide

Rodents

Molluscicide

Snails

Algacide

Algae

insecticides (32%), fungicides (7%), and other categories (6%) (8).

Insecticides comprise compounds of various chemical classes; some of the most common are listed in Table 2. Insecticides have a variety of mechanisms of action in insects, including affecting the insects' metabolism (nerve poisons, muscle poisons, dessicants, and sterilants) or through a physical effect such as clogging air passages. The most common classes of insecticides are chlorinated hydrocarbons, organophosphates, and carbamates. The chlorinated hydrocarbons DDT and aldrin, among others (Table 2), were developed in the 1930s and 1940s. Such pesticides were very potent (high insect toxicity), but presented simultaneously chronic health and environmental effects because of their resistance to environmental and metabolic breakdown, which led to widespread environmental contamination and buildup of residues in a variety

Table 2. Some Pesticide Classes and Examples of Each Class

Pesticide

Examples

Insecticides

Chlorinated hydrocarbons Organophosphates

Carbamates Pyrethroids

Triazine Phenoxy

Quaternary ammonium Benzoic acids Acetanilides Ureas

Dicofol, methoxychlor, DDT,"

aldrin," dieldrin," chlordane" Parathion, malathion, phosdrin, diazinon, chlorpyrifos, azinphos-methyl

Aldicarb, carbaryl, carbofuran Permethrin, Cypermethrin

Herbicides

Atrazine, cyanazine 2,4 D Paraquat Dicamba

Alachlor, metolachlor Linuron

Fungicides

Inorganic

Ethylenebisdithiocarbamates Chlorinated phenols

Sulfur

Maneb, mancozeb Pentachlorophenol

"Banned in the United States.

of animals, including humans. Most chlorinated hydrocarbons are no longer permitted for use in the United States, although a few of the least persistent members of the family are still allowed. Historically, the carbamate and organ-ophosphate insecticides served as replacements for many of the chlorinated hydrocarbons. These insecticides exert their toxicity on both insects and mammals through inhibition of cholinesterase enzymes that normally function to regulate nervous system activity. Although more acutely toxic to nontarget organisms, including mammals, the or-ganophosphate and carbamate insecticides are less persistent in the environment. Another class of insecticides, the pyrethroids, were introduced in the 1970s. They are considered excellent broad-spectrum insecticides; they are effective at low doses, exhibit low toxicity to mammals, and break down quite rapidly in the environment. Pyrethroids continue to be used extensively in agriculture but also have the disadvantages of being relatively costly and environmentally labile, and they commonly lose their effectiveness due to the development of insect resistance.

Herbicides include a variety of chemical compounds that act upon weeds through different toxicological mechanisms. In some cases the toxicity results from direct plant contact by destroying leaf and stem tissues. Other herbicides inhibit seed germination or seedling growth; damage leaf cells, causing weeds to dry up; or affect the weed's ability to perform photosynthesis (2).

Fungi can cause damage or stress to food crops, and massive infestations can occur during storage of many foods, including grains, when storage occurs under conditions of high temperature and/or moisture. A serious consequence of fungal attack can be the production of myco-toxins, such as aflatoxins and fumonisins, by Aspergillus flavus and Fusarium moniliforme, respectively. Some fungicides can destroy fungi that have already invaded the plant while others may prevent fungal infestations.

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