Fumigating Soils and Plants

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. Row^s 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 stonefruit 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 hotwater 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.