Because of their extreme toxicity to warm-blooded animals, these potent insecticides may not be used to control insects affecting man and animals, such as household pests and cattle and sheep pests. Because they are effective at extremely low dosages against a wide range of insect species, and when properly applied leave a negligible spray residue on an agricultural crop, their potential usefulness is great.

SYNTHETIC ORGANIC CHEMICALS have been used as fumigants for nearly a century. They are low-boiling compounds of rather simple structure. They include hydrocarbon derivatives that contain sulfur, oxygen, chlorine, bromine, and nitrogen. Carbon disulfide (CS₂), prepared now from sulfur and coke by heating in an electric furnace, was the pioneer. Ethylene oxide, (CH₂) ₂O, is a gas at ordinary temperature and was proposed as a fumigant in 1928. One part is used with 10 parts of carbon dioxide to reduce the fire hazards.

Among the chlorinated hydrocarbons we find carbon tetrachloride, CCl₄, used with ethylene dichloride, C₂ H₄,Cl₂, (1:3) since 1927; propylene dichloride, C₃H₆C1₂; ethylene dichloride alone; and a mixture of 1, 2-dichloropropane, C₃H₆C1₂, and I,3-dichloropropylene, C,H,Cl,, known as D-D. These materials and certain bromine compounds methyl bromide, CH₃,Br, and ethylene dibromide, C₂H₄Br₂ are of value against wireworms and nematodes.

Hydrocyanic acid, or hydrogen cyanide (HCN), is a highly poisonous gas used in fumigation of citrus trees as well as a space fumigant for warehouses and other enclosed places.

Some compounds of higher boiling point and more complex structure are also used as fumigants because of their high vapor pressure. Chloropicrin, CI₃CNO₂, boils at 112.4 C. and is used as a fumigant for grain and soil. It is most effective in a mixture of I pound to I gallon of carbon tetrachloride.

Dichloroethyl ether (C₂,H₄,CI) ₂ O, bis (2-chloroethyl) ether with a boiling point of 78.5 C. produces vapors much heavier than air and is of value as a soil fumigant.

Naphthalene, C,,H,, is one of the older organic insecticides not obtained from plants or oil. It is a hydrocarbon obtained by the destructive distillation of coal. This flaky white solid has been Used for a half century to protect woolen cloth against clothes moths. It has a fumigating action, but its objectionable odor is not easily removed from the fabric. The newer moth-proofing materials do not have that disadvantage.

p-Dichlorobenzene, C₆H₄,Cl₂, a white, odorous solid, which melts at about 53 C., has wide use for control of peach tree borers and clothes moths. It is synthesized by reacting chlorine with benzene in the presence of the proper catalysts. It is one of the best known fumigants because of its long and wide usage.

A desire to find materials that would increase the toxicity and thus extend the supply of scarce insecticides, such as pyrethrum, has encouraged investigation in this field. Such materials are known as synergists. N-isobutylundecylenamide, N-isobutvlhendecenamide, the first synergist developed for pyrethrum, was introduced in 1938. It may be considered a synthetic material, although castor oil is the basic material for its preparation. The value of sesame oil as a synergist for pyrethrum was discovered about the same time. Its effectiveness was shown to be due to the presence of sesamin. Knowledge of the structure of sesamin led to the synthesis of related compounds, including piperonyl cyclonene and piperonyl butoxide.

Piperonyl butoxide, also known as ( butyl carbitol) (6-propylpiperonyl) ether, is a thick, viscous liquid that contains as its principal active constituent a-[2 - (2 - butoxyethoxy) ethoxy] - 4,5- rnethylenedioxy- 2-propyltoluene. Piperonyl cyclonene, formerly known as piperonyl cyclohexenone, is the common name for a mixture comprised of 3-alkyl - 6 - carbethoxy - 5 - (3,4-mcthy1enedioxyphenyl) - 2-cyclohexen- 1 -one and 3 - alkyl - 5- (3,4 - methylenedioxyphenyl)-2-cyclohexen-1 -one, in which the "alkyl" refers to aliphatic radicals that may be varied. It is a thick, viscous liquid. Another synergist for pyrethrum is n-propyl isome, the dipropyl ester of 1,2,3,4-tetrahydro - 3-methyl-6,7-methylenedioxy- 1,2 - naphthalenedicarboxylic acid. Like sesamin, the three materials all contain the methylenedioxyphenyl group.

One of the later organic insecticides, N-octylbicyclo[2.2.1]-5-heptene-2,3-dicatboximide, MGK 264, was introduced as a synergist for pyrethrins but has also been found to be effective as an ovicide. It is an amber-colored and rather viscous liquid. It is slightly heavier than water. It is readily soluble in the usual organic solvents and is itself a good solvent for quite a few of the other newly discovered insecticides.

Oil of citronella, a plant product, was the standard repellent for mosquitoes before the Second World War. During the war, however, the need for repellents for chiggers, mosquitoes, and fleas instigated the testing of many synthetic organic compounds, Benzil, C₆H₅COCOC₆H₅, and benzyl benzoate, C₆H₅COOCH₂C₆H₅, were found to be repellent to chiggers; dimethyl phthalate to mosquitoes and mites; Rutgers 612 (2-ethyl-1,3-hexanediol), Indalone (often called n-butyl mesityl oxide oxalate but more properly the butyl ester of 3,4-dihydro- 2,2-dimethyl4-OXO-2H-pyran-6-carboxylic acid ) and dimethyl carbate ( the dimethyl ester of cis-bicyclo [2.2.1]-5-heptene-2,3- dicarboxylic acid) to mosquitoes, chiggers, and fleas. A mixture of Indalone, dimethyl phthalate, and Rutgers 612 is used as an all-purpose insect repellent.