OIL AND WATER, despite the old saying, can be made to mix in the form of emulsions wherein the oil is dispersed as minute droplets throughout the water. Oil is usually applied to fruit trees in the form of emulsions containing about 1 percent to 4 percent oil. Emulsification is brought about by agitation and the addition of a substance, known as an emulsifying agent, that reduces interfacial tension.
Oils are applied as emulsions primarily to regulate the amount of oil deposited on the plant. That is important: A rather direct relationship exists between oil deposit and both insecticidal efficiency and plant injury. The object is to lay down a deposit sufficient to kill the pests present and yet below that which will cause plant injury. Often the operational margin is quite narrow. Oil-deposition rate is determined chiefly by four factors: The oil strength in the spray mixture, the kind and amount of emulsifying agent used, the nature of the plant surface sprayed, and the amount of spray applied.
The first requirement of an emulsifying agent, of course, is that it produce a satisfactory emulsion. It also should maintain a uniform concentration of oil throughout the batch of dilute emulsion in the spray tank. These conditions can be met by forming highly stable emulsions. Unfortunately stable emulsions generally lay down low oil deposits in spraying. To obtain at least moderate-deposition properties in the mixture, one must sacrifice some stability. Actually, agitation can largely offset this disadvantage. Most modern spraying machines are equipped with agitation systems that permit the use of relatively unstable emulsions.
The influence of the emulsifier on oil-deposition rate in spraying may be great. An emulsion prepared with one emulsifier may lay down as much oil on the plant at a 1-percent strength as others used at 2-, 3-, or even 4-percent strengths. Further wide variations in deposition can be expected as the amount of any given emulsifier is varied. The deposition rate for a given emulsifier generally decreases as the amount used is increased.
Another factor is the nature of the plant surface whether bark, leaves, or fruit, or, indeed, old and new bark, young and mature fruit, old and new leaves, and often the upper and lower surfaces of leaves. The surface factor is of less importance in treating deciduous fruit trees during the semi-dormant period, when relatively small variations in bark surface are involved. The treatment of trees in leaf is something else again. The surface factor is of special importance in treating citrus trees for the control of pests like the California red scale, which occurs on all parts of the tree. If a pest must be controlled on two or more types of surfaces, the amount of emulsifier should be so adjusted that at least a minimum effective dosage will be laid down on all surfaces.
Thus, any recommendations for oil sprays should consider the concentration of oil in the spray mixture and its oil-deposition rate as well. There has been a trend towards adjusting deposition rates to common standards.
Probably no single oil-deposition standard will prove satisfactory for all purposes. When the oil deposit must be rigidly controlled, as in spraying oil-sensitive shade trees during dormancy and deciduous fruit trees in leaf, a relatively stable light-depositing emulsion is indicated. But deciduous fruit trees are relatively tolerant of oil in the dormant period. Some overdosing of all or part of the tree then would be of little importance. Consequently less stable emulsions may then be used on fruit trees.
Growers can buy a stock oil product in which the emulsifying agent is incorporated or buy the straight oil and emulsifier separately and prepare the emulsion themselves in the spraying machine immediately before use. The latter is called tank mixing. Satisfactory spray-strength emulsions can be prepared by tank mixing as well as through the use of commercial stocks. Tank mixing costs less, but factory-made formulations offer convenience in handling and uniform performance.
Commercial spray-oil stocks are Of two classes, concentrated emulsions and emulsible oils. Such terms as emulsive oils, miscible oils, and soluble oils are also applied to the second type. Concentrated emulsions preformed emulsions in a concentrated state resemble a thin, whitish paste and usually contain about 83 percent oil by volume. The concentrated emulsions will flow readily through the standard 2-inch bung for metal drums.
The emulsible oils consist of oil in which one or more emulsifying agents have been dissolved. They usually contain 95 to 99 percent oil and often resemble straight oil in appearance. They are not emulsions in the state in which they are sold but produce emulsions when added to water in the spray tank. They vary in the readiness with which an emulsion is formed in the tank. Some formulations produce an emulsion instantly; others first require some preliminary agitation in the presence of only a small amount of water. Some authorities prefer to designate the former type of product as miscible oils, reserving the term emulsible oil for the latter. Although the so-called miscible oils emulsify readily, they lay down low oil deposits in spraying.
The tank-mixing procedure is quite simple in principle. A 2-percent oil spray mixture can be prepared thus, in a high-pressure orchard-spraying machine equipped with a 400-gallon tank: With the engine running, just enough water is drawn into the tank to operate the pump-15 to 25 gallons. The emulsifying agent is added, then the oil (which would be 8 gallons in this example). A spray gun directed into the tank is next opened and held open for 1 to 2 minutes. The circulation of water, oil, and emulsifier through the pump and its discharge or injection under high pressure into the tank effects emulsification. At this point the mixture should have a uniform, creamy appearance. The final step is to fill the tank with water, and the mixture is ready for use.
The foregoing procedure will produce the most satisfactory type of tank-mixed emulsion, but it is not absolutely necessary to pass the mixture through a spray gun. In the citrus area of California a general practice is to wait a minute or two before filling the tank for the agitators to create the emulsion. An improvement on the practice is to operate the pumps under full pressure during the prefill mixing.
Many emulsifying agents may be used in tank mixing. Blood albumin has been widely used. In California a 25-percent product is used at the rate Of 4 ounces for each loo gallons of spray-strength emulsion. An 8-ounce rate is advised in New York.
Application of more of the oil-spray mixture than may be needed to cover all or part of the tree usually causes no harmful effects. There is a limit to how much oil can be deposited in continuous spraying when most dilute emulsions are used. It simply runs off beyond this point. An important exception is when part of a tree may be sprayed twice with a drying period between. The situation may occur when growers follow the practice of spraying one side of the row when the wind, say, is in the west, and covering the east side several days later when the wind shifts. Almost twice as much oil will be deposited where the two coverages overlap as elsewhere on the tree. One should try to cover the whole tree in one operation. If each side of the row is sprayed separately, the opposite side should be treated 15 or 20 minutes later, or before the spray applied in the first half of the operation has dried.
PETROLEUM-OIL SPRAYS have been used on citrus trees since about 1900. Commercial control of the major pests in most California citrus districts can be had with a single annual application of an oil spray. Such a program, the most economical of those available, has been widely followed in California. The dominant position of oil sprays on citrus is being challenged as the search continues for more efficient insecticides and for ones without the objectionable effect on trees and fruit that is attributed to oil.
Different practices are followed with oil sprays on citrus in Florida because of differences in climate, cultural practices, varieties, and in marketing. Florida citrus trees are apparently more tolerant of oil sprays than are those of California at least there seems to be greater latitude in the kinds of oil that can be used with relative safety on citrus in Florida. Growers in Florida in 1945 were using oils that ranged in viscosity from 69 to 108 seconds Say-bolt at 100 F. and from 75 to 92 percent in, U. R. Both naphthenic- and paraffinic-type products were employed. Practices in California were more standardized; the spray oils were prepared from much the same class of crude (California naphthenic-base) stock, refined to a U. R. of go percent or higher, and were available in a series of relatively narrow boiling fractions.
The California Department of Agriculture in 1932 established specifications for spray oils that were based on certain U. R. and distillation standards. The latter property was measured as the percentage of the product that distilled up to 636 F.
Five grades of oils were established for use on citrus fruit trees light, light-medium, medium, heavy-medium, and heavy. The minimum required standards for each grade are given in the accompanying table. Kerosene and mineral-seal oil are also included because they have sometimes been applied on citrus.