Oil Sprays for Fruit Trees

P. J. Chapman, L. A. Riehl, G. W. Pearce.

Petroleum oils are used in several ways to control pests. Some kill insects and mites directly through their own action. Some supplement the action of other insecticides as co-toxicants, solvents and carriers, stickers, or stabilizers.

In the water-borne oil sprays commonly applied to fruit trees, the oil usually is the sole or primary insecticidal agent. That is also true of oils used to rid bodies of water of mosquitoes.

Light petroleum fractions are widely used as solvents and carriers for many insecticides. The original fly sprays are a good example. The introduction of DDT and other organic insecticides has meant a great increase in the use of oil as the carrier for applying insecticides, especially the chemicals used to control household and building pests. These oil-insecticide mixtures usually are applied in the form of fine mists. With heat and a suitable generator they can be applied also as thermal fogs, which remind one of military smoke screens.

Often oils are added to insecticidal and fungicidal spray, dust, and poison-bait formulations as stickers, stabilizers, and conditioning agents.

In this chapter we discuss the waterborne oil sprays as they are used to control pests of citrus and deciduous fruit trees.

Kerosene was apparently the first petroleum product used for the control of plant pests in the United States. A. J. Cook of Michigan State College introduced in 1877 a kerosene-soap emulsion which was widely employed to combat aphids and scale insects.

Entomologists sought something more effective and turned to crude petroleum. It proved to be too injurious to most plants. A search was then started for some fraction or series of fractions of petroleum that would be highly effective as insecticides, but relatively noninjurious to plants. Progress has been made in the search.

Oil sprays are used most commonly in horticulture to control scale insects and mites, among which are many of our major fruit pests. Oil sprays are also used to control psyllids ( pear psylla), plant bugs ( apple red bugs) , mealybugs, aleyrodids (whiteflies, citrus blackfly) , thrips, aphids ( newly hatched), membracids (buffalo tree-hopper), and others. Oil sprays readily destroy eggs of many lepidopterous pests, like the codling moth, oriental fruit moth, various leaf rollers, and cankerworms. Those insects are now more commonly controlled in the larval stage with the newer insecticides.

More than 15 million gallons of oil are used annually in this country for horticultural sprays. Emulsified and diluted to a 2-percent strength, that amount makes 750 million gallons of spray enough to provide for the single coverage of 40 million to 5o million orange or apple trees.

Tree spray oils are of two classes. Those intended for use on the hardy tree fruits during the dormant period are called dormant oils. Those applied to trees in foliage are the summer oils. The oils used on citrus in California may be classed as summer oils. The two groups differ chiefly in the degree of refinement of the oil and in its heaviness, or viscosity. Summer oils have been more highly refined and are of lighter weight than dormant oils. The classification is rather arbitrary, and because the trend has been toward using the so-called dormant oils after growth starts and using more highly refined products, the distinction between dormant and summer oils has had less and less meaning.

The first major step in refining petroleum is its division into fractions by distillation. First to distill over are the low-boiling naphthas, then come increasingly higher-boiling lots, through gasoline, kerosene, fuel oils, and, finally, the lubricant fractions. Horticultural spray oils are derived from the fuel-oil and light-lubricant portions of petroleum; those from the lubricant portion predominate.

Crude petroleums vary greatly in composition. Differences exist among crudes from the major production fields and even among wells in one field. We recognize three general types paraffinic base, asphaltic or naphthenic base, and mixed base or midcontinent crudes. Spray oils have been prepared from all crude classes. Asphaltic-base crudes are utilized in California primarily because the local petroleum supply is generally of that class. East of the Rockies the midcontinent crudes are more commonly used.

Before we consider specifications for horticultural spray oils, it is well to have an understanding of their nature.

The spray oils are composed essentially of hydrocarbons compounds containing hydrogen and carbon. The arrangement of the atoms of the two elements in individual molecules is varied and complex. Only three basic classes of carbon structures occur, however paraffin chains, aromatic rings, and naphthene rings. It is possible by analysis to determine the approximate percentage of each structure-class in any oil. As will be brought out later, oil composition has an important bearing on both insecticidal efficiency and plant safety. The composition one might find for spray oils, manufactured from paraffinic and naphthenic crudes, is:

Research workers learned long ago that the safeness of spray oils to plants in leaf is related to the aromatics and other unsaturates present. It is now generally agreed that oils can be made increasingly safer for use on evergreen plants and on deciduous plants in their growth period by lowering the aromatic-ring content. That may be accomplished in part in refining operations by treating the oil with strong sulfuric acid or its equivalent. The aromatics and other unsaturated structures react to form sulfonates, which can be separated from the remainder of the oil. The process has given rise to the term unsulfonated residue, or U. R. The term is widely used to indicate the degree of refinement of the oil or its degree of freedom from aromatic structures. Oils intended for foliage sprays have U. R. values ranging from about go to 96 percent. Products used on deciduous trees in the dormant period may range from about 50 to 90 U. R.

Until 1940 or so, oil composition was thought to have little practical relation to the insecticidal efficiency of horticultural spray oils. Since then, however, studies made at the New York State Agricultural Experiment Station and elsewhere have established that efficiency is related to the paraffinicity of an oil. Thus efficiency increases as the paraffinic character in oils increases. The relationship has been demonstrated in the case of the major oil-susceptible pests of both deciduous and citrus fruit trees. It should not be inferred that the so-called naphthenic oils make unsatisfactory spray oils. They are in a sense simply low paraffin oils and consequently are used at greater strengths in the spray mixture to achieve results equal to those had with highly paraffinic items.

Another factor affecting the insecticidal value of an oil is the size of its molecules. More familiar but less accurate terms for this property are viscosity and relative heaviness : Oils of small molecular size, such as kerosene, for example, have little separate value in killing horticultural pests, but there seems to be no advantage in going above a certain molecular size.

VISCOSITY AND BOILING-RANGF, data are the criteria most commonly used in commerce to indicate the molecular-size property of an oil. Viscosity is measured at certain temperatures by recording the time required in seconds for a sample to flow through a standard opening. It depends on the principle that molecular size controls flow speed, with the rate decreasing as size increases. In addition to size, however, flow rate is also affected by the shape of the molecules. That means that one cannot depend on viscosity measurements alone to classify products as to their suitability for horticultural sprays among oils of different origin. For example, a highly paraffinic oil having a viscosity of only 50 seconds Saybolt at 100 F. may be more effective insecticidally than extremely naphthenic products of 125-130 seconds. Viscosity measurements are useful in indicating heaviness ranges among oils of common origin and manufacture.

A more accurate indication of molecular size in an oil can be obtained from distillation- or boiling-range data. Moreover, that measure has two advantages over viscosity data for spray-oil purposes: It indicates molecular-size range, and it permits a fairly close practical comparison, insecticidally, even among oils of different composition.

The California Department of Agriculture in 1932 established distillation-range standards to regulate the sale of spray oils in that State. Five grades of summer or foliage-spray oils and three grades of dormant oils were set up, based on a minimum U. R., and the percentage of the product that distilled over at 636 F. The system has not been adopted generally, but it has worked out well on the west coast, partly because spray oils in the area are made from the same general class of crude petroleum.

OIL SPRAYS KILL insects and mites by what appears to be essentially a smothering action. By enveloping the pest with a continuous film of oil, the oil interferes with its respiration and ultimately causes death. That is the conclusion to be drawn from studies made by E. H. Smith and G. W. Pearce, who used eggs of the oriental fruit moth as test subjects. That the action is largely physical was shown by the ability of some eggs to survive 24-hour exposure to a lethal dosage of oil. In the experiment the oil was removed 24 hours after application through the use of an oil solvent.

Similar results were obtained earlier by this technique on the winter eggs of the fruit tree leaf roller. In that instance, some eggs hatched after having been exposed a week to a deposit that would have killed all eggs had it not been removed.