Since ancient times there have been intermittent records of various substances being used to destroy insects, most of them based on folklore or on such writers as Pliny. Few of these nostrums could have been reliable, however, because they were often based on specious logic rather than on reason and experience. Thus, parts of a plant having a fancied resemblance to a noxious pest were often believed to be a specific for destroying it. During the last hundred years, however, there has been steady progress in the development of really effective insecticides. This period can be roughly divided into three phases, the first of which extended up to the Second World War. The substances which were in use at the beginning of this phase were those known from general experience to be toxic to most forms of life. They included crude inorganic chemicals such as arsenicals and mercury compounds and plant products such as nicotine and derris. Pyrethrum was a fortunate exception, which happened to be specifically active against insects. Then, to such more or less natural products, were added various by-products of the growing chemical industry; especially those from coal tar and petroleum distillation. Later, attempts were made to increase potency by chemical modifications; by introducing chlorine or thiocyanate radicles. Meanwhile, organic chemists had identified the active principles of pyrethrum and derris, so that these vegetable insecticides could be standardized; but synthesis was not then feasible.
KeywordsScrub Typhus Hydrogen Cyanide Wettable Powder Carbamate Insecticide Organochlorine Insecticide
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