¹ In steel bins the dosages may be reduced 50 percent for small grains and about 20 percent for corn and grain sorghum.

² The addition of 5 percent by volume of ethylene dibromide improves the kill of immature stages of insects in grain.

In former years a general fumigation once or twice a year with hydrocyanic acid, methyl bromide, or chloropicrin was relied on to keep premises free from insects. Modern demands for food entirely free from insect infestation have caused the adoption in many plants of a biweekly program of local fumigation, whereby individual milling units or food-handling machines are fumigated and the fumigated stock removed by heavy-duty vacuum cleaners. In some mills the biweekly application of local fumigants takes the place of a general fumigation. Local fumigants, if regularly used, will maintain a low insect population in milling machinery but cannot be expected to destroy infestations in all parts of the plant, so that an occasional general fumigation is helpful.

General fumigants are usually introduced into the open space of the building, but sometimes are also introduced through piping systems directly into the machinery.

Local fumigants may be applied by hand by pouring them into the individual milling units or machines. Liquid fumigants may also be applied with permanently installed dispensers. Portable fumigant dispensing tanks are used to force the fumigant into fabric tubes installed permanently inside conveyors or other units. A fully automatic system dispenses the fumigant in vapor form from a central supply connected by tubing to individual milling units.

chloropicrin, hydrocyanic acid, and mixtures of carbon tetrachloride with ethylene dibromide, ethylene dichloride or other chemicals are used extensively as local fumigants.

For the fumigation of warehouses filled with grain, feed, flour, or other dried foodstuffs, methyl bromide or mixtures of methyl bromide with chloropicrin or ethylene dibromide have been found most effective. Perfect penetration of large stacks of bagged materials can be obtained with dosages of 1.5 pounds per 1,000 cubic feet of space. To obtain uniform distribution of the fumigant and to prevent stratification of the vapors near the floor, electric fans should be operated for I hour after release of the gas.

THE USE OF TARPAULINS or gas-proofed fabrics in the fumigation of stored foodstuffs sometimes may be more convenient than fumigating in large, partly filled warehouses or in atmospheric vaults. The tarpaulin, which takes the place of the fumigation chamber, is portable and occupies little space when not in use. The free air space is reduced to a minimum and aeration is facilitated by the complete removal of the tarpaulin from the stack of commodities after fumigation. The products to be fumigated are generally stacked on a concrete floor and covered completely by the tarpaulin, the edges of which are weighted down carefully to prevent leakage of gas around the base. Provision is made for an air dome at the top by using two sacks placed edgewise about 4 feet apart. The air dome will provide free air space to permit diffusion of the gas.

A rubberized fabric or a light duck material coated with ethyl cellulose usually is used. Any fumigant suitable for the treatment of bulk commodities in atmospheric vaults or warehouses can be used to treat foodstuffs under tarpaulins.

Atmospheric vaults are useful for the fumigation of foodstuffs when warehouses are not tight enough for efficient fumigation or when small lots incoming raw materials, returned goods, used bags, and out-going products of all kinds need treatment. Many different materials can be used to construct atmospheric vaults, but a metal vault or one with a metal lining is most efficient. Tubing and spray nozzles for introducing volatile fumigants and fans for circulating the fumigant or exhausting the vapors are necessary.

The actual process of fumigation is simple. The commodity is loaded into the vault by hand or run in on trucks or skids. The door is closed and the fumigant introduced. At the end of the fumigation, the exhaust fan is turned on and allowed to run until the vapors, not absorbed by the fumigated commodity, have been removed. While the vault is being unloaded, the exhaust fan should be kept running. Sometimes auxiliary fans may be needed to supply fresh air for the workmen unless they wear gas masks.

Methyl bromide and mixtures of methyl bromide with chloropicrin or ethylene dibromide are most efficient for use in treating dried foodstuffs in atmospheric vaults, although hydrocyanic acid, chloropicrin, and many other fumigants can be used. Dosages depend on the commodity to be fumigated., the quantity involved, and the fumigant.

Fumigation by vacuum consists of placing the commodity in a gas-tight steel chamber, removing the air, and replacing it with a gas lethal to insects. By this method a more rapid penetration of commodities by the gas is obtained than in atmospheric fumigation, and insects are reached and killed faster than in an atmospheric vault. The removal of a large part of the oxygen from the chamber makes the insects more susceptible to fumigants. The length of exposure ranges from 1 to 3 hours, compared to 10 to 24 hours under atmospheric conditions; an important factor in industries where speed is essential in handling foodstuffs.

Vacuum fumigation has several other advantages. At the end of a fumigation, the removal of the fumigant from the treated commodities can be speeded up by a process known as air washing. It consists of drawing a vacuum of 27 inches or more and breaking it with air. There is little danger that workmen will enter a vault undergoing fumigation, and the danger from breathing the vapors during the unloading of a vault is lessened.

It is advantageous to draw as high a vacuum as possible and to hold the vacuum throughout the exposure. By circulating the gas in the tank for 15 minutes after it is introduced, the distribution of the fumigant will be aided greatly, and much less fumigant will be needed to effect a kill than if the gas is not circulated.

The fumigants usually employed in vacuum vaults are methyl bromide, hydrocyanic acid, and a 1-9 mixture of ethylene oxide and carbon dioxide.

Dosages vary with the fumigant, the commodity, and the length of the exposure. The shorter the exposure, the larger the amount of the fumigant required.

The fumigation of individual packages of foodstuffs is practiced in some food industries, but is expensive because comparatively large quantities of fumigant must be used. In this method, the individual packages traveling along a belt pass under an applicator, which automatically injects a certain amount of fumigant into each one. The packages are then sealed. Each package is its own fumigation chamber. The method was first used extensively in the United States to treat packages of dried fruit, for which ethyl formate, methyl formate, and isopropyl formate have been used. Other products so fumigated are dried soup-stocks, rice, dog biscuits, popcorn, and such. Besides the formates, acrylonitrile in admixture with carbon tetrachloride has been used for individual packages.

R. T. COTTON has been an entomologist in the Bureau of Entomology and Plant Quarantine since 1919 and has been in charge of field research on the control of the insect pests and stored-grain and milled-cereal products since 1934. He has specialized on fumigation and other methods of controlling the insect pests of stored foodstuffs. Dr. Cotton holds degrees from Cornell University and George Washington University.