Chemical Structures of Insecticides
Insecticides belong to both categories of chemicals: inorganic and organic substances. Nowadays, inorganic insecticides—arsenicals, fluorosilicates, hydrogen cyanide, hydrogen phosphide, bromides, etc., are not much used in India. However, hydrogen cyanide (HCN) occurs in crops as natural toxin. At the same time, agricultural scientists feel that arsenicals could be advantageously used against leaf-feeding insects, particularly parasites or predators. The Food Safety and Standards Regulation (FSSR) 2011 permits MRL of 37.5 and 25 ppm in food grains for HCN and inorganic bromide, respectively. At the same time, FSSR 2011 does not permit hydrogen phosphide even in residual traces (FSSR 2011). Among the organic insecticides, there are a few substances of bacterial and plant origin: nicotine sulphate, pyrethrins, rotenone, etc., as well as a large number of synthetic insecticides. Organic insecticides are categorised as hydrocarbons, organo- nonmetallics and organometallics.
In general, organo-nonmetallic insecticides include organophosphorus and organosulphur compounds, while organometallic insecticides include mercury, tin, copper and zinc compounds. The FSSR 2011 permits 148 insecticide residues with prescribed MRL or tolerance limits for particular food articles. However, it also mentions hydrogen phosphide with nil tolerance limit (FSSR 2011).
Organic insecticides, extensively used for crop protection, cover the wide range of chemical structures, including hydrocarbons, carboxylic acid derivatives, alcohols, aldehydes, ketones, amines, nitro compounds, quinones, thiocyanates, mercaptans, heterocyclic compounds, etc. However, a four-class subdivision may be proposed: organochlorines, organophosphorus, carbamates and pyrethrins/ pyrethroids (Buchel 1983), despite a wide range of chemical structures.