By Ed Wright
Many or the chat lines have gotten pretty well keyed up about the use of oil to control pests on plants. Sides have been taken and seconds appointed on everything from axle grease to baby oil, but few facts have been offered. I have about as few facts as anybody, so let me share them with you.
Horticultural oils are not new. They have been around for at least a century. In earlier times, oils and kerosine were mixed with a surfactant such as laundry soap and sprayed on dormant plants. The surfactant helped oil disperse in water and also caused it to spread or sheet in a thin film over plants, insects, pest eggs, fungi, etc. When the water evaporated, the remaining film suffocated many pests and diseases. Since the plants were dormant, damage to them was minimal.
Unfortunately, films from dormant sprays persist on plants and slowly build up into a significant layer. As sun energy builds up in the spring, plants break dormancy and dormant spray films begin to melt. Soon, pest and plant alike are suffocated or burned by oil spray residue. We refer to this effect as phytotoxicity: literally, poisonous to plants.
In relatively recent times, chemists have discovered that very carefully controlled refining techniques can produce horticultural oils with a vastly reduced tendency toward phytotoxicity. These oils are now so finely controlled they can be applied year around within critical limitations. First, the oil must be "pulled off" the refining stream within a very narrow temperature range. Each manufacturer has a secret temperature for this, but all are probably very close to 412 degrees Fahrenheit. This is toward the low end for the "paraffinic" oils typified by kerosine. Gasoline is in the next lower group while the next higher group is termed "light gas and oils". Mixtures also vary by brand, but a 4% solution might be considered typical.
Next, horticultural oils must be mixed and applied exactly as recommended. Each brand has a very precise evaporation point for the oil and a carefully designed spread for the surfactant. Combined, evaporation point and spread control film thickness and tenacity. With modern oils,the film stays on just long enough to control the target pest, then it washes off, oxidizes, orvolatizes (or all of the above) as it is removed from the plant. Result: virtually no phytotoxicity.
Bottom line: Horticultural oils are inexpensive, effective and environmentally kind. Why play oil roulette with your plants? Properly used, horticultural oils are kind to us, easy on our plants and pests do not develop immunity to them. Like you, I hear self-appointed experts recommending "any old cooking oil" or salad oil, mineral oil, baby oil and, for all I know, used motor oil. I shudder when I think of the consequences to plants. Horticultural oils are a precision tool for pest management. Let's use the proper oil in the proper way and properly appreciate those who employ so much care to bring them to us.
