Mode of Action of Phytotoxic Oils

TN AREAS near oil refining centers, as in most of California, petro1 leum oils are widely used in agriculture. Reference is not made here to the use of petroleum to power and lubricate tractors and other farm machinery, but to its use as an agricultural chemical. There are three typical uses of oils: (1) insecticidal spray oils, (2) various oils as selective and general herbicides, and (3) solvent type oils as carriers for oil soluble agricultural chemicals. In California alone, annually from five to fifteen million gallons of petroleum oil are used for the control of scale, mites, and mealy bug (23). An estimated 10 million gallons of cracked gas oils are used for general weed control on canal banks, road sides, barnyards, and in orchards practicing non-tillage. Much solvent type oil is also used for selective weed control in umbelliferous crops, and for pre-emergence weed control in lettuce, beets, and onions. One large operator alone is using one million gallons annually. Aromatic fractions and fortified heavier oils are used as drying agents on flax, clover, and alfalfa seed crops and as cotton defoliants. DDT and other insecticides are commonly applied as dilute oil emulsions. Principal reasons for the use of oils as herbicides are that they are especially suitable for the elimination of weedy grasses (4), and that when properly applied, they do not harm crop plants by toxic residues in the soil. Previous to 1930, no special weed oils were made, unaltered diesel and smudge oil were used (23). When the diesel oils were improved as fuel by additional refining, they became less suitable as herbicides (5). At present weed oils are special oils selected to serve the purpose for which they are intended. However, it must be remembered that these special oils are not by-products in the sense that they are waste products without other uses, but are made available only in lieu of their use as fuel oil and solvents. Therefore, the availability of weed oils is inversely related to the demand for fuel oils and commercial solvents but the price relationship is normally a direct one. Distance from the source of supply also is an important factor because of shipping costs.