INSECTICIDES act to kill insects by a number of mechanisms, of which the principal ones will be discussed here.
Organochlorine insecticides, these include the original simple organochlorine compound DDT (chlorophenothane) introduced in the 1940s, and its successor methoxychlor. Later commonly used compounds were the chlorinated cyclodienes, including dieldrin, aldrin, heptachlor and chlordane. Other chlorinated hydrocarbon insecticides include lindane and the extremely persistent agents mirex and chlordecone. The uses of these in agriculture and industry and the toxicology of these agents are outside the scope of this article. Lindane and dieldrin can be used in medicine as acaricidal or scabicidal agents (see ACARICIDES). A number of organophosphorus ANTICHOLINESTERASES have been developed for use as insecticides, and have largely replaced the organochlorine compounds (which are generally very persistent). They are developed from agents originally designed for warfare, and loosely referred to as nerve gases’ (e.g. dyflos, tabun, sarin and soman). Insecticides derived from this series include TEPP (ethyl pyrophosphate, an early agent), parathion, paraoxon (active metabolite of parathion), fenthion, malathion and dimpylate (diazinon). Alhough safer for the environment, they require great care in handling. In fact, parathion is the insecticide most often involved in fatal poisoning.
Carbamate anticholinesterases. these are ‘reversible’ in as much as their duration of action is short as compared to organophosphorus anticholinesterases, and are used extensively. An example is carbaryl (carbaril) and several analogues of carbaryl are used as insecticides. However, not all carbamates found in garden formulations are cholin-esterase inhibitors; the dithiocarbamates are fungicidal.
Plant-derived insecticides, for example, pyrethrum, which is a crude extract from flowers of the pyrethrum plant Chrysanthemum cincerariaefolium. Pyrethrin is a more refined extract containing the six naturally occurring pyrethrins. The greatest activity resides in pyrethrin I. Pyrethrum is regarded as one of the safest insecticides, at least in terms of primary toxicity. It resembles DDT in mode of action, and works at least in part by opening sodium channels in excitable membranes, so causing paralysis. See SODIUM-CHANNEL ACTIVATORS.
Rotenone (derris) is obtained from the roots of plants such as Derm and Lonchocarpus, and was used to paralyse fish before being developed into an insecticide. Human poisoning is rare, and it has been used to treat head lice and other ectoparasites.
Nicotine is one of the most toxic of insecticides in terms of acute poisoning, but in dilute solution or suspension is an effective acute insecticide. See NICOTINIC CHOLINOCEPTOR AGONISTS.