Despite the dangers, hydrocyanic acid is widely used. For some purposes it is measured from cylinders with the aid of air pressure. For others it is generated by addition of its sodium or potassium salts to a mixture of sulfuric acid and water. For special uses it is packaged with an absorbent such as felt or diatomaceous earth. The gas is released on distribution of the absorbent in the space to be fumigated. It also is released on distributing granular calcium cyanide in the presence of moist air.

Its first important use as a fumigant was developed in California in the 1880's for the control of scale insects on citrus trees. At first the gas was generated under a tent covering the tree. Liquid hydrocyanic acid, made available since, has become the most important source of the fumigant. The method can be used without serious injury to the trees in the drier parts of California, but it has not been successful in more humid areas, such as Florida and Louisiana.

It long has been used as a fumigant for plants in greenhouses. Such fumigations, and those of citrus trees, are made at night to reduce the hazard of plant injury; during daylight hours, plants absorb the gas more readily. The plants must be dry when fumigated, or the gas will dissolve in the moisture on the plant and may cause injury.

Houses, warehouses, flour mills, storage vaults, and ships are often fumigated with hydrocyanic acid to destroy insects. It is very important that all be thoroughly aired out before anyone enters them. Things like mattresses and pillows require long airing. It is absorbed so rapidly that it may not penetrate throughout large bulks of milled cereals. Fresh fruits and vegetables, dried fruits, grain, flour, and other foodstuffs have been fumigated with hydrocyanic acid. Most of those that have hard rinds or skins do not absorb dangerous amounts of the fumigant. Others, such as green vegetables, immature potatoes, or bananas, may be injured severely. After airing, the amounts absorbed from ordinary fumigation concentrations and exposures are not considered to be unduly hazardous to humans.

METHYL BROMIDE, or bromomethane (CH₃Br), is a colorless gas at room temperatures. It is almost odorless. Its boiling point is 4.6 C., melting point 93 C., specific gravity 1.732 0 /0 C., and vapor pressure 760 millimeters at 4.6 C. Its vapors are about 3.3 times as heavy as air.

It is non-inflammable. Its vapors mixed with air cannot be ignited by a flame. It is therefore useful as a fire extinguisher. Specific conditions can be established to provide for ignition by means of an intense electric spark, but they are not encountered during ordinary fumigations.

Methyl bromide is toxic to humans. Prolonged breathing of its vapors should be avoided. The lack of a warning odor makes it especially dangerous. Poisoning, which may not be apparent until hours or a day after exposure, may result from breathing its vapors. A gas mask should always be worn when exposure is possible. In mild cases the symptoms are disturbance of the equilibrium, double vision, headache, and vertigo. In acute cases delirium, loss of consciousness, convulsions, and sometimes death occur.

Its use as a fumigant has increased rapidly since 1932, when its usefulness was first reported. Its many advantages make it one of the most widely used of all fumigants.

It is highly toxic to many kinds of insects in all stages of development. It is chemically stable, is only slightly soluble in water, and (at the concentrations required) has no deleterious effect on most plant tissues. It imparts no objectionable taste. or odor to food- stuffs, and usually leaves no dangerous residue. It is convenient to handle, for it is readily liquefied, and yet it vaporizes at temperatures encountered in fumigating.

The action of methyl bromide on insects may be slow. Certain species, such as larvae of the Japanese beetle, may show little evidence of being affected after fumigation; death may occur a week or more later. With other fumigants, such as hydrocyanic acid, death generally occurs during fumigation or shortly thereafter or the apparently dead insect may recover.

Some of the important uses of methyl bromide are for the fumigation of warehouses, flour mills, ships, and residences, and flour, grains, seeds, fruits, vegetables, tobacco, and bulbs in vaults, bins, tanks, or railroad cars. It is also highly effective as a soil fumigant; often it is used to treat nursery stock infested with certain insects against which quarantines have been established. Besides, it is used to rid many varieties of potted greenhouse plants of various insect species.

All plants and vegetables will not tolerate methyl bromide fumigation. A few varieties of azaleas and evergreens are apt to be injured. The ripening of tomatoes may be delayed and sweetpotatoes may spoil.

NAPHTHALENE (C₁₀H₈) is a white crystalline compound at room temperatures. It has a strong, characteristic odor. Other properties : Melting point 80.22 C., boiling point 217.9 C., density 1.145, vapor pressure 0.08 millimeter at 20 C. Its vapors are about 4.4 times as heavy as air.

Its vapors burn with a luminous but smoky flame. It is one of the safer fumigants with reference to explosion hazard. Because specific mixtures of its vapors and air can be ignited, it should not be used near open flames.

Under ordinary fumigating conditions, naphthalene is not dangerous to humans. Its strong odor at high concentrations and the irritating effect of its vapors on the eyes and nose offer ample warning to prevent the breathing of injurious amounts. Prolonged breathing of its vapors may cause a delirious condition. If the crystals are ingested, the symptoms include excessive vomiting, purging, and great abdominal pain, followed by nephritis.

The main use of naphthalene as a fumigant is for the protection of woolen goods and furs, and it is commonly known as moth balls or moth flakes. Paradichlorobenzene in cake or crystal form is often used in place of it.

It has been used to a limited extent as a soil fumigant for the control of wireworms or the larvae of certain insect species, such as the Japanese beetle. Newer and more effective fumigants include ethylene dibromide and methyl bromide.

NICOTINE, or 1-methyl-2- (3 -pyridyl) pyrolidine (C₁₀H₁₄N₂), is the chief alkaloid of tobacco. It is an oily, colorless liquid. It is almost odorless when it is pure but develops a tobacco-like smell on standing and rapidly turns brown when it is exposed to air. Other properties : Boiling point 247.3 C., melting point below 80 C., specific gravity 1.00093 20 /4 C., vapor pressures 0.08 millimeter at 20 C. and 7.00 millimeters at 100 C. Its vapors are about 5.6 times as heavy as air.

As far as fire and explosion dangers are concerned, nicotine is one of the safer fumigants. Only 0.04 pound of it is required to saturate 1,000 cubic feet of air at 68 F. Much lower concentrations are required to kill certain insects. It is inflammable, so it is best to avoid using it near open flames.

Nicotine is one of the most virulent poisons known to man. Serious or fatal poisoning may result from ingestion of very small amounts of it, from absorption through the skin or from breathing its vapors. Its strong odor, very irritating to the nose, provides warning of its presence during fumigations. If it is spilled on the skin, it should be washed off immediately. It causes contraction of the blood vessels with marked increase in the blood pressure, followed, after larger doses, with vascular dilation and fall. of pressure. The pulse rate is lowered at first and later becomes rapid.

The principal use of nicotine as a fumigant is for the control of insects in greenhouses. It was first used for the purpose about 1825, tobacco being burned in an enclosed space. Later it was volatilized from water solutions, or the alkaloid was heated and the vapors distributed by means of a fan. Nicotine aerosols have been highly effective. Very likely the effectiveness of nicotine in sprays and dusts applied to trees and other plants is due partly to fumigation and partly to absorption or ingestion.

SULFUR DIOXIDE (SO₂) is a colorless gas at room temperatures. It has a strong, characteristic odor. Its boiling point is 10 C., melting point 72.7 C., and liquid density 1.434. It is about 2.2 times as heavy as air.

It is non-inflammable and can be used without danger from fire or explosion.

It is highly toxic to humans, but its vapors at low concentrations are so irritating to the eyes, nose, and throat that acute poisoning is rare. At high concentrations it is absorbed by the moist surfaces of the respiratory tract and results in inflammation and swelling. A person is usually unable to remain long enough in a space being fumigated with sulfur dioxide to receive a toxic dose.

It is one of the first known of the fumigants, but other fumigants have largely taken its place. It is soluble in water, forming sulfurous acid, which is corrosive, and is a powerful bleaching agent. Under moist conditions, it is apt to tarnish metals and cause injury to colored wallpaper and fabrics. In moist or dry climate it is toxic to many insects. It has long been used as a home fumigant or in other locations where the hazards mentioned are not important or are preferable to the presence of the insects. It is a practical fumigant for refrigerator cars in which fresh fruit is shipped. It may be distributed in enclosed spaces by burning sulfur or by releasing it from commercial cylinders.

Sulfur dioxide is injurious to growing plants, many kinds of fruits and vegetables, and wheat and flour for bread making. It adversely affects seed germination.

OTHER FUMIGANTS used in limited amounts to meet specific insect-control problems include acrylonitrile, 1,1 -dichloro-1-nitroethane, 1,1-dichloro-1-nitropropane, ethyl formate, methallyl chloride, methylene chloride, methyl formate, propylene dichloride, and tetrachloroethane.

ROBERT D. CHISHOLM is a chemist in charge of the Moorestown, N. J., laboratory of the division of insecticide investigations, Bureau of Entomology and Plant Quarantine. After graduation from the University of Massachusetts, he was engaged for about 15 years as a chemist in the commercial manufacture of insecticides and fungicides. He joined the Department in 1935. Much of his work has been on the development of insecticides, attractants, and equipment for the control of the Japanese beetle and of bark beetles.

Wireworm.

House fly larva.