Major Classes of Conventional Insecticides
There are more than 400 specific chemical compounds that are currently registered for use in the United States as insecticides. They represent a wide range of chemical structures, toxicity, and physical properties. Some are general purpose insecticides, while others have very specific and limited uses. In addition, there are many other compounds that have been used in the past but are no longer available because of high toxicity, long persistence, or low reliability. The following list, organized by chemical families, includes some of the more important insecticides — both past and present.
Inorganic Compounds
Arsenic and fluoride compounds were the mainstay of chemical pest control from the mid 1800’s to the mid 1900’s. These materials [e.g., lead arsenate, arsenic trioxide, and copper acetoarsenate (Paris green)], are persistent in the environment and highly toxic to all forms of animal life. Most inorganic compounds were phased out of use after World War II. They were replaced by synthetic organic compounds that were more effective and less hazardous to humans and the environment. Sodium fluoride (NaF) and cryolite (sodium fluoroaluminate, Na3AlF6) are stomach poisons that still have commercial applications. Finely ground sulfur is also used as both a fungicide and a miticide.
Source: Tobacco (Nicotiana spp.) — Solanaceae. Sold commercially as a fumigant (nicotine) or as a dust (nicotine sulfate).
Source: Tropical legumes — Leguminaceae. Derris spp. grown in Malaysia and the East Indies and Lonchocarpus spp. grown in South America
These were the first synthetic organic insecticides ever discovered. The group includes DDT and its relatives (e.g., methoxychlor and kelthane), lindane, toxaphene, and cyclodiene insecticides (e.g., aldrin, dieldrin, endrin, mirex, chlordane, heptachlor, and endosulfan). Most organochlorines (they are also known as chlorinated hydrocarbons) are relatively stable compounds that act as nerve poisons. They are several thousand times more soluble in fat (lipid) than in water. This means they tend to accumulate in fatty tissues and concentrate in organisms at the top of a community’s food chain (bioaccumulation). Most uses of organochlorines have been banned or discontinued in the United States because of environmental problems associated with their long persistence and bioaccumulation. Lindane is still used as a seed dressing and sold by prescription for control of human lice; endosulfan and kelthane are still used for agricultural pest control.
These compounds were first discovered by German scientists in the late 1930’s as a by-product of nerve gas research. They are much less persistent than the organochlorines and do not accumulate in fatty tissues. The group includes many general purpose insecticides with a wide range of mammalian toxicity (e.g. malathion, parathion, diazinon, chlorpyrifos, azinphosmethyl, acephate, phorate, and phosmet) as well as compounds that work as fumigants (e.g., DDVP) and as systemics (e.g. dimethoate, disulfoton, demeton, and ronnel). All of the organophosphates are nerve poisons. They block the active site of an enzyme (acetylcholinesterase) that breaks down and removes a neurotransmitter substance (acetylcholine) from the nerve synapse. The excessive build up of acetylcholine results in symptoms of hyperactivity, including tremors, convulsions, and eventually death.
First developed in the early 1950’s, this large family of insecticides is comparable in many respects to the organophosphates, sharing a common mode of action, biodegradability, low solubility in fat, and a wide range of mammalian toxicity. Carbaryl (Sevin) is probably the best known and most widely used carbamate; it also the least toxic to humans. Other carbamates include carbofuran, propoxur, methomyl, bendiocarb, formetanate, oxamyl, and aldicarb (the last three are plant systemics). One advantage carbamates have over organophosphates is reversibility of their inhibitory reaction with acetylcholinesterase. Chronic exposure to carbamates, therefore, is less likely to cause illness than chronic exposure to organophosphates.