Spraying and dusting, in agriculture, the standard methods of applying pest-control chemicals and other compounds. In spraying, the chemicals to be applied are dissolved or suspended in water or, less commonly, in an oil-based carrier. The mixture is then applied as a fine mist to plants, animals, soils, or products to be treated. In dusting, as an alternative method, dry, finely powdered chemicals may be mixed with an inert carrier and applied with some type of blower. Dry granular materials also have been used instead of dusts or sprays where adequate coverage can be obtained. (See also crop duster). In fumigation, gases or the vapours of volatile compounds are held in contact with the materials to be treated—grain in a tight bin, for example.
The development of more effective sprays and dusts and their increasingly widespread use in agriculture have prompted concern among biologists and others that humans may disrupt nature and endanger the food and water supply and public health. New chemicals and new precautions have only partially reduced these dangers.
Sprays and dusts are used to control insects, mites, and fungous and bacterial diseases of plants; insects, such as lice and flies, on animals; and weeds, by means of chemical weed killers or herbicides. Sprays and dusts may also be used for such special purposes as applying mineral fertilizers, increasing or decreasing fruit set, delaying the dropping of nearly mature fruits, and defoliating and vine killing to facilitate the harvest of such plants as cotton or potatoes.
Sprays have advantages over dusts in their ability to adhere to and spread over treated surfaces. Spreading-sticking agents or surfactants are commonly added to spray mixtures to increase adhesion and wetting of waxy surfaces. These wetting agents decrease the tendency of water to collect in drops and permit the chemical solution to spread over a leaf in a very thin film, bringing the spray chemical into maximum contact with the fungi, bacteria, insects, or mites to be controlled. There has also been an increase in the use of air sprayers or “wet dusting.” The use of a concentrated spray distributed in the airsteam from a powerful fan combines many of the advantages of both spraying and dusting.
Fumigation may be used to control insects and some diseases in stored products or to control insects such as wireworms and grubs, nematodes, and sometimes fungi and weeds in soil. The chemical may be applied as a gas or as a volatile liquid. Partial fumigation of the soil may be accomplished by applying the chemical in a spray or granular material and immediately working it into the soil. For better pest control on small areas like seedbeds the treated soil is covered with gastight, plastic cover.