Insecticide Country Groups

Organophosphates (OPs) are a group of chemicals with similar structures and similar modes of action, which includes the nerve gases. Only the pesticides wiU be considered in this chapter. Organophosphates were developed during the Second World War as insecticides and chemical warfare agents. The first was parathion, which is very toxic compared to DDT and to other OPs and is no longer used in most countries (see box and Table i). 

Turning to the worldwide situation, herbicides are again the single most important group of pesticides. However the sales value of insecticides is not far behind and reflects the severe problems which insects cause in climates warmer than our own temperate one. Not only is there interference with growing crops (e.g. by locusts) but in tropical countries insects are responsible for the transmission of several virulent diseases. Malaria is perhaps the best known of these, and programmes for its eradication have represented a major outlet for insecticides. 

Insecticides are the second most important group of pesticides on a worldwide basis and are of prime importance in tropical countries where their use (measured in monetary terms) may exceed that of all other groups of pesticides put together. 

In 1939, the highly efficient contact insecticide DDT (an abbreviation of the technically incorrect name dichlorodiphenyl-trichloroethane) was introduced to the market. Its use, as well as the use of several other organochlorine compounds, spread worldwide in the following years. Important active substances that appeared on the market in the period after World War II include carbamate insecticides and herbicides. Also significant was the discovery of herbicidal phenoxyacetic acids, which represent the first group of the so-caUed hormonally active pesticides. Around the middle of the last century a number of other biologically active substances were discovered, many of which, such as herbicides based on substituted urea, s-triazines (1,3,5-triazines), quaternary ammonium salts or insecticidal synthetic pyrethroids and many others, are still used in many countries around the world. 

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