The number of chemicals used as pesticides

It is very interesting to study lists of pesticides for sale in 1945 or earlier. Lead arsenate, mercury salts, and some organic mercury compounds, zinc arsenate, cyanide salts, nicotine, nitrocresol, and sodium chlorate were sold with few restrictions. Very few of these early pesticides are now regarded as safe. The world had a very strong need for safe and efficient pesticides like DDT. This fantastic new substance started to appear on the lists of approved pesticides under various names (Gesarol, Boxol S, pentachlo-rodiphenylethane, etc.) at that time. 

The herbicide 2,4-D got a similar status as the first real efficient herbicide that made mechanization in agriculture possible. The discovery of 2,4-D as an herbicide during World War 2 precipitated the greatest single advance in the science of weed control and the most significant in agriculture (cited in Peterson, 1967). 

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