The term agricultural chemicals has largely been replaced by the term ‘pesticides’ defined as economic poisons that are used to control, kill, or repel pests. Depending on what a compound is designed to do, pesticides have been sub classified into a number of categories. The primary classes of pesticides in use today are fumigants, fungicides, herbicides, and insecticides.
Organochlorine Insecticides
The chlorinated hydrocarbon insecticides include familiar insecticides such as DDT, methoxychlor, chlordane, heptachlor, aldrin, dieldrin, endrin, toxaphene, mirex, and lindane. DDT, as well as other organochlorines, were used extensively from the 1940s through the 1960s in agriculture and mosquito control, particularly in the World Health Organization (WHO) malaria control programs. The cyclodiene insecticides, such as chlordane were used extensively as termiticides into the 1980s but were removed from the market due to measurable residue levels penetrating into interiors and allegedly causing health problems. Residue levels of chlorinated insecticides continue to be found in the environment and, although the concentrations are now so low as to approach the limit of delectability, there continues to be concern.
Organophosphorus Insecticides Organophosphorus pesticides (OPs) are phosphoric acid esters or thiophosphoric acid esters and are among the most widely used pesticides for insect control. Parathion was another widely used insecticide due to its stability in aqueous solutions and its broad range of insecticidal activity. However, its high mammalian toxicitythrough all routes of exposure led to the development of less hazardous compounds. Malathion [diethyl (dimethoxythiophosphorylthio) succinate], in particular, has low mammalian toxicity because mammals possess certain enzymes, the carboxyl esterases,that readily hydrolyze the carboxyester link, detoxifying the compound. Insects, by contrast, do not readily hydrolyze this ester, and the result is its selective insecticidal action.
Botanical Insecticides Extracts from plants have been used for centuries to control insects. Nicotine [(S)-3- (1-methyl-2-pyrrolidyl)pyridine] is an alkaloid occurring in a number of Plants. Nicotine is quite toxic orally aswell as dermally. Symptoms of acute nicotine poisoning occur rapidly, and death may occur with a few minutes. In serious poisoning cases death results from respiratory failure due to paralysis of respiratory muscles. In therapy attention isfocused primarily on support of respiration.
Pyrethrin is an extract from several types of chrysanthemum, and is one of the oldest insecticides used by humans. There are six esters and acids associated with this botanical insecticide. Pyrethrin is applied at low doses and is considered to be nonpersistent. The most frequent reaction to pyrethrins is contact dermatitis and allergic respiratory reactions, probably as a result of other constituents in the formulation. Synthetic mimics of pyrethrins, known as the pyrethroids, were developed to over comethe lack of persistence.
Herbicides control weeds and are the most widely used class of pesticides. This class of pesticide can be applied to crops using many strategies to eliminate or reduce weed populations.
Fungicides fungi cause crop losses In addition recent studies have shown that toxins and other airborne organic compounds released from fungi inhabiting the interior of dwellings probably are responsible for a number of adverse health effects. Compounds produced to combat these losses and adverse health effects are called fungicides, and a number of these families have been around for years. The fungicide, chlorothalonil (tetrachloroisophthalonitrile), is a broad-spectrum fungicide which is used widely in urban environments. It is relatively cheap and controls some 140 species of organisms. As a result of the popularity of this compound, it is found routinely in surface waters entering public drinking water supplies. In the formulation that can be purchased by the general public, it is relatively nontoxic.
Rodenticides This class of compounds is used to control rodents. As the rats navigate through narrow passages, they bruise themselves, developing small hemorrhages. Anticoagulants prevent the blood from clotting, and the animals bleed to death in about a week. Humans who are exposed to this class of compounds are given vitamin K, and if the poisoning is severe, blood transfusions as a treatment.
Conclusion: Today’s modern pesticide is generally safe to use if the directions on the label are followed. Advances in instrumentation and an understanding of how adverse health effects are produced have resulted in the production of many environmentally friendly but effective pesticides.