An insecticide is a pesticide used against insects in all developmental forms. They include ovicides and larvicides used against the eggs and larvae of insects respectively. Insecticides are used in agriculture, medicine, industry and the household. The use of insecticides is believed to be one of the major factors behind the increase in agricultural productivity in the 20th century. Nearly all insecticides have the potential to significantly alter ecosystems; many are toxic to humans; and others are concentrated in the food chain. It is necessary to balance agricultural needs with environmental and health issues when using insecticides.

Classes of agricultural insecticides

The classification of insecticides is done in several different ways:

• Systemic insecticides are incorporated by treated plants. Insects ingest the insecticide while feeding on the plants.
• Contact insecticides are toxic to insects brought into direct contact. Efficacy is often related to the quality of pesticide application, with small droplets (such as aerosols) often improving performance.
• Natural insecticides, such as nicotine and pyrethrum, are made by plants as defences against insects. Nicotine based insecticides have been barred in the U.S. since 2001 to prevent residues from contaminating foods.
• Inorganic insecticides are manufactured with metals and include arsenates copper- and fluorine compounds, which are now seldom used, and sulfur, which is commonly used.
• Organic insecticides are synthetic chemicals which comprise the largest numbers of pesticides available for use today.
• Mode of action  how the pesticide kills or inactivates a pest  is another way of classifying insecticides. Mode of action is important in predicting whether an insecticide will be toxic to unrelated species such as fish, birds and mammals.

Heavy metals, e.g. lead, mercury, arsenic, as well as plant toxins such as nicotine have been used for many years. Various plants have been used as folk insecticides for centuries, including tobacco and pyrethrum. Some farmers are reporting successfully using spray of crudely fermented alcohol as an effective insecticide.

Organochlorine compounds

The insectcidal properties of the best known representative of this class of insecticides, DDT, was made by the Swiss Scientist Paul Műller. For this discovery, he was awarded the Nobel Prize for Physiology and Medicine in 1948. DDT was introduced on the market in 1944. With the rise of the modern chemical industry it was possible to make chlorinated hydrocarbons. DDT works by opening the sodium channels in the nerve cells of the insect.

Organophosphates

The next large class developed was the organophosphates, which bind to acetylcholinesterase and other cholinesterases. This results in disruption of nervous impulses, killing the insect or interfering with its ability to carry on normal functions. Organophosphate insecticides and chemical warfare nerve agents (such as sarin, tabun, soman and VX) work in the same way. Organophosphates have an additive toxic effect to wildlife, so multiple exposures to the chemicals amplifies the toxicity.