Fumigating Soils and Plants

Randall Latta, M. C. Lane.

Fumigants differ from other types of insecticides in that the fumes must be confined so that the insect is exposed to a considerable concentration for some time. The length of exposure and the strength of the concentration are interrelated the higher the concentration, the shorter the lethal exposure; the lower the concentration, the longer the exposure.

Fumigants therefore are not well adapted for controlling insects on growing crops. They are often the only efficient method for treating plant commodities, however, when the insect is protected within seeds, pulp, or stems or is in the soil around the roots and when the treatment must be effective within hours so that the commodity can be moved into commerce.

Fumigants serve three general purposes : To treat growing crops, to destroy insects in soil, and to treat plant commodities.

An example of the use of a fumigant on a growing crop is the treatment of citrus trees for controlling scale insects and other citrus pests. Hydrocyanic acid gas, HCN, has long been utilized for that purpose. Rows of trees are covered with tents. The dosage, in proportion to the tree size, is injected or blown under the edge of each tent in the row, and the trees are exposed to the fumes for an hour. The tents are moved to the next line of trees, and the process is repeated. The fumigation is carried on when there is little or no air movement, usually in the late evening. The method has been adopted for the control of Hall scale, an insect on stone-fruit trees.

Fumigation of growing plants in greenhouses and mushroom houses is an old practice. HCN evolved from granular calcium cyanide and gas evolved from nicotine compounds heated, burned, or painted on hot-water pipes are often used. Many other fumigants have been tried, but none has been so widely accepted as those two.

Organic phosphate insecticides such as hexaethyl tetraphosphate (HETP), tetraethyl pyrophosphate (TEPP), tetraethyl dithiopyrophosphate, and parathion and other organic materials, such as lindane, can be applied as aerosols in greenhouses to give a combined contact and fumigation effect. The vapors from these materials are toxic to insects in extremely low concentrations.

FUMIGANT VAPORS may be retained for a long time in the soil, and they might be quite toxic to insects and other organisms living there. Probably the first such fumigant to have widespread use was carbon disulfide. It was employed to kill the grape phylloxera, a root-louse that was threatening the grape and wine industry in France. The chemical was tested against wire-worms in the United States as early as 1891 and was recommended for use against various soil insects until recently. Paradichlorobenzene ( applied in crystalline form in soil around tree trunks to control the peach borer) and napthalene flakes ( worked into surface soil for wireworms) were other early soil fumigants. Chloropicrin, calcium cyanide, and many other fumigants have been used in attempting to control soil insects and nematodes. Most have been too costly or too difficult to apply to be practical on any large scale on the farm. All have limited use in greenhouses or seedbeds.

Because the damage caused by wire-worms, symphilids, and nematodes is so great, many fumigants have been tested for use in soil. In some years, wireworm damage to the potato crop of the Pacific Northwest alone has caused losses of 4 million dollars to farmers. Soil pests have caused losses to the lima bean crop of several million dollars annually for many years in California. The damage to other crops probably has been proportionally as great.

During the late 1930's and early 1940's research for better fumigants was intensified. Many new organic chemicals came on the market. One, a mixture of the two chemicals dichloropropane and dichloropropylene and known as D-D, was used in 1943 against the pineapple mealybug in Hawaii and was found to be a potent agent against mealybugs and nematodes. Later tests in California demonstrated its effectiveness against wireworms. Another material. dichloronitroethane, proved to be more effective and suitable when soil temperatures were low.

Ethylene dibromide was found to be an efficient and economical fumigant for wireworms.

An increased interest in soil fumigation has led to an improvement in testing procedures. Until recently only the fumigants that had proved successful for fumigating grain or households were tested, and much the same testing methods were followed. Research workers later began testing fumigants in the presence of soil instead of exposing the insect alone. The newer tests disclosed that more fumigant was required when it was applied in soil, that it must be active enough to move around freely in the soil, and that it must not be too strongly absorbed by the soil.

Soil physicists have investigated the movement of gases in the soil. Some of their work can be applied to soil fumigation. The scientists agree that gases enter and leave and move around in the soil mass by diffusion. Diffusion is slow, especially in compact soil.

Experiments revealed that dichloronitroethane moved in compact soil about 24 inches in 16 days, or at an average rate of 1.5 inches a day. The rate is increased when the soil is loosened; the movement depends on the amount of free air space. Harrowing, rolling, or anything that reduces the free air space slows down the rate of diffusion. Plowing and disking, which loosen the soil, increase the rate of diffusion. The amount of water in the soil also influences diffusion, as water fills the air spaces and slows down the movement of gases. Experiments in 1949 showed that 10 to 30 times the amount of fumigant actually needed to kill wireworms must be applied to field soil for successful control. This large excess of fumigant is absorbed by the soil or escapes into the air.

The best and most widely used soil fumigants are ethylene dibromide and a mixture of dichloropropane and dichloropropylene. Both have been used successfully against wireworms and nematodes.

Ethylene dibromide, a heavy liquid with a rather low rate of evaporation, moves slowly through the soil. Its rate of escape from the surface also is slow. It should be used in loose soil to speed diffusion. Some sort of surface seal (such as provided by a light rolling or harrowing with a spike harrow) is desirable. It is about as efficient in cold soils, down to nearly freezing, as in warm soils. It is not greatly affected by soil moisture if the soil is not saturated with water. The wireworms common in the Pacific Northwest can be controlled by a dosage of about 2 gallons of ethylene dibromide to the acre. That amount is actually diluted with a highly refined light oil, such as paint thinner, because the available equipment does not readily measure or apply less than 6 to 8 gallons an acre. Larger doses are needed to control nematodes.

The dichloropropane-dichloropropylene mixture, also a liquid, is much lighter than ethylene dibromide and not nearly so toxic to insects. About 25 gallons to the acre are required to control wireworms in the Pacific Northwest. It is much more volatile than ethylene dibromide, and the soil surface must be sealed by harrowing or rolling after the fumigant is applied. Neither fumigant should be used in saturated soil. They should be given a week to 10 days to permeate the soil and kill the insects. Thereafter, if the odor of the fumigant is still strong in the soil, heavy disking or spring toothing will open it up and allow the fumigant to escape.

With these new and more practical fumigants has come the development of machines for applying them. The old hand-operated, single-row injection machines of earlier days were impractical on the large acreages that needed treatment in the Western States and Hawaii.

The many different machines that have been used are mostly of the power-injection or gravity-feed types. For the larger acreages the trailer- or tractor-mounted types are satisfactory. They can cover 10 to 40 acres a day. The liquid fumigant is released into the soil under pressure through tubes fastened to the rear of soil-chisel shanks mounted on draw bars so that the fumigant is injected at the best depths for maximum penetration. The chisel shanks are usually set 12 inches apart, and there may be 5 to 14 on a machine according to the power available to pull them through the soil. Some drawbacks of injection machines are their high cost, their inability to work well except on a prepared soil bed, and the rapid escape of the gas through the apertures left by the chisel shanks. Thousands of acres have been treated since 1945 with the machines, using D-D and ethylene dibromide.

The gravity-feed applicators are more suitable for the average farmer on a small acreage. Also known as plow applicators, they can be made on the farm from a second-hand gasoline tank, some quarter-inch copper tubing, and a valve or two attached to a standard tractor or plow. The fumigant is discharged by gravity just ahead of the plow or plows onto the exposed plow sole, where it is covered immediately by the soil of the next furrow. Needle valves regulate the flow according to the speed of the tractor and width of the furrow. The equipment is low in cost. The soil does not have to be prepared beforehand. If the surface is harrowed lightly after plowing, the toxic vapors are held in the soil long enough to give the most efficient diffusion of the fumigant.

Lindane and parathion are effective for treating soil to destroy insects in greenhouses and plant nurseries where plants are growing. Small amounts of the materials added to soil in the greenhouse bench will control symphilids. The effect is a combination of contact and fumigation.

OFFICIALS who enforce plant quarantines are interested in soil fumigation that will free restricted areas from a particular insect so that plants can be grown or stored there without hazard of infestation and subsequent dissemination of the pest.

Under the Japanese beetle quarantine, several methods were perfected. One is fumigating under tar paper or tarpaulin covers with carbon disulfide injected in holes 1 foot apart each way. Another is sprinkling a water solution of methyl bromide or a mixture of ethylene dibromide and ethylene dichloride over the soil surface.

A third is treating the soil that would make the ball of a balled and bur-lapped nursery plant. Before the plant is dug, a quantity of emulsions or solutions containing carbon disulfide, methyl bromide, or a mixture of ethylene dibromide and ethylene dichloride is applied to the area around the plant in the nursery row. A modification of the method is to dip the soil or root balls of nursery plants after digging in emulsions or solutions of carbon disulfide, ethylene dichloride, a mixture of ethylene dibromide and ethylene dichloride, or a mixture of a fumigant and a contact insecticide ethylene di-bromide and chlordane.