The Organic Insecticides

C. V. Bowen, S. A. Hall.

The best known of the synthetic organic insecticides is DDT, but it was not the first. Some of them have been in use for decades. Carbon disulfide, p-dichlorobenzene, and naphthalene stand out as old-timers. Ethylene dichloride, ethylene dibromide, methyl bromide, and thiocyanates have been used for the past quarter century. Thousands of similar compounds man-made materials whose basis is carbon have been investigated as to insecticidal value. The Department of Agriculture in 1922 or so began a study of their use as repellents and fumigants and began later the synthesis of materials for testing as poisons for insects.

Phenothiazine, thiodiphenylamine, introduced as an insecticide in 1935, may be considered one of the early members of the newer synthetic age. It is used now to only a limited extent as a codling moth insecticide, but it is used extensively for the internal medication of livestock for the control and removal of injurious nematodes that infest cattle, horses, sheep, and goats.

Azobenzene, an orange crystalline material, was found in 1943 to be effective as a fumigant for the control of mites in greenhouses. Because azobenzene sublimes readily, a solution containing it may be applied to steam pipes and allowed to vaporize. The development of the organic phosphorus compounds, however, has greatly lessened its use.

A group of dinitro derivatives of phenol and cresol came into use before the Second World War as dormant sprays in apple orchards. The simplest of these, 4,6-dinitro-o-cresol, DNOC, formerly known as 3,5-dinitro-o-cresol, is a solid melting at 85.8 C. and not very soluble in water. The sodium derivative (called a salt) often is used in the dormant sprays because of its greater solubility in water. Analogs similar substances in which the methyl group of the cresol has been replaced by cyclohexyl or by some other group are also used. Salts other than sodium, such as the dicyclohexylamine or triethanolamine salt, are also in use.

Aliphatic, alicyclic, and aromatic esters of thiocyanic acid were used before the Second World War in household and horticultural sprays. The so-called lauryl thiocyanate is a mixture of compounds containing alkyl groups derived from the natural fatty acids of coconut oil in which the lauryl, or 12-carbon, chain predominates. Other thiocyanates used as insecticides are the 2-(2-butoxyethoxy) ethyl ester of thiocyanic acid, diethylene glycol di-ester of thiocyanic acid, /3-thiocyanoethyl esters of aliphatic fatty acids averaging from 10 to 18 carbon atoms, and isobornyl thiocyanoacetate. Because some of them may injure growing plants, care should be exercised in using them.

IN RECENT YEARS a great deal has been said about chlorinated hydrocarbons as insecticides. The use of this type of compound is not new, for carbon tetrachloride and p-dichlorobenzene, which have been used for years, are chlorinated hydrocarbons.

DDT is a chlorinated hydrocarbon insecticide. The raw materials for its manufacture are chlorine, benzene, and alcohol. DDT was first described in 1874 by a German chemist, Othmar Zeidler, but its insecticidal value was not discovered until about 1939 by Paul Muller, in Switzerland. It was first introduced into the United States in August 1942 when the dye firm of J. R. Geigy shipped from Switzerland to New York two formulations a dust and a wettable powder for testing by American entomologists. Later undiluted DDT was imported, and in June 1943 the manufacture of DDT was begun in the United States for use by the Armed Forces. When the end of the Second World War made DDT available for civilian use, it came into large-scale use as an insecticide.

The symbol DDT combines the first letters in the name dichloro-diphenyl-trichloroethane. The precise chemical name for the principal toxic ingredient of technical DDT is 1,1,1-trichloro-2,2-bis(p-chlorophenyl) ethane. A discussion of its chemistry was presented in Science in Farming, the Yearbook of Agriculture for 1943-1947. DDT is unstable in the presence of alkalies and consequently is not compatible with alkaline agricultural chemicals. It is also decomposed by iron and some iron salts.

Benzene hexachloride, or 1,2,3,4,5,6- hexachlorocyclohexane (i. e., BHC, HCH or HCCH), is a chlorinated hydrocarbon made by reacting chlorine with benzene in the presence of ultraviolet light to produce a compound with the molecular formula which the English designated as 666. The material was first made by Michael Faraday in 1825, but its insecticidal action was not known until many years later. Harry Bender, an American chemist, in a United States patent application for a method of chlorinating hydrocarbons, mentioned in 1933 that the benzene hexachlorides appeared to be good insecticides, but apparently no use was made of the idea. A. P. W. Dupire in France in 1941 applied for a French patent on the use of benzene hexachloride as an insecticide based on entomological tests conducted in 1940. In 1942 a sample of benzene hexachloride made by F. D. Leicester in England was found to be insecticidal, and the compound came into use in that country in place of derris in flea beetle powder.

Technical benzene hexachloride is made up of a mixture of isomers, compounds that are identical in chemical structure except for a difference in the orientation in space of some of the constituent atoms. F. J. D. Thomas in England in 1943 found that the insecticidal principle of technical benzene hexachloride was the gamma isomer. The isomers had been named alpha, beta, gamma, and delta in the order In which they had been isolated, alpha and beta by F. E. Matthews in 1891 and gamma and delta by T. von der Linden, a German chemist, in 1912. In 1949, the common name lindane was selected for the gamma isomer of benzene hexachloride of not less than 99 percent purity after von der Linden. The gamma isomer comprises about 12.5 percent of crude benzene hexachloride. Because of its odor and the off-flavor it imparts to certain food products, technical benzene hexachloride is limited in use. Lindane, however, is practically odorless. Investigations were started in 1947 to check its effect on the flavor of fruits, vegetables, and meats. It is a white crystalline solid, soluble in most of the common organic solvents but insoluble in water. It has some fumigant properties and is a contact and stomach poison.

In the early study of DDT, analyses of the technical material revealed 4 percent of an impurity that has insecticidal properties. The impurity was identified as 1,1-dichloro-2,2-bis(pchlorophenyl) ethane, a byproduct of the reaction used in making DDT. The compound has been referred to as DDD and as TDE, from its generic names dichloro- diphenyl - dichloroethane and tetrachloro - diphenylethane. It is closely related to DDT in chemical structure and properties. It will react with alkalies and consequently should not be formulated with alkaline materials.

Another of the analogs of DDT has been given the common name methoxychlor because it has a formula in which two of the chlorine atoms of DDT have been replaced by the methoxy group (CH₃O ). Like DDT, technical methoxychlor (which contains about 80 percent of 1,1,1-trichloro-2-2- bis (p-methoxyphenyl) ethane) also is a white solid, soluble in the common organic solvents and insoluble in water. It is less effective than DDT against most insects but is less toxic to warm-blooded animals. Alkaline materials promote decomposition of methoxychlor and consequently must not be used in its formulations.

Analogs of DDT containing bromine and fluorine have been tested for insecticidal action. 2,2-Bis(p-bromophenyl) - 1, 1, 1-trichloroethane sometimes has been referred to as Colorado 9. 1, 1, 1 - Trichloro - 2,2-bis (p-fluorophenyl) ethane is a constituent of an insecticide which the Germans called Gix. Although insecticidal in action, neither material has come into commercial use in the United States, probably because their cost is greater than that of DDT.

Toxaphene, C,,H,,Cl,, is the common name for a product obtained by reacting chlorine with camphene. It is more complex than benzene hexachloride. Its structure is not completely known. The technical material consists of a mixture of compounds, which contain 67-69 percent chlorine. It was originally known as Hercules 3956, but a more descriptive name is chlorinated camphene. It is a cream-colored solid of waxy consistency. It melts over a range of 65 to 90 C. Toxaphene is readily soluble in the common organic solvents. Toxaphene will dehydrochlorinate in the presence of alkalies. Like DDT, it slowly splits off hydrochloric acid on heating and in the presence of materials, such as iron compounds, that may act as catalysts.