Horticultural oil sprays have been used for more than a century on fruit trees to control insects and disease. Traditionally, these sprays, aimed at killing overwintering insects, were applied in late winter while trees were dormant. Today, with improved oil-refining techniques, gardeners can take advantage of lightweight, high-quality oils suitable for combatting pests and disease, on many trees, vegetables and flowers throughout the growing season. These updated formulas are variously referred to as summer, superior or supreme oils.
Horticultural oils act against insects in several different ways. Most importantly, they block the pests' breathing pores (spiracles), eventually causing asphyxiation. Additionally, the oil can penetrate insects' thin membranes and eggs, leading to cell death. Treatments also interfere with the ability of some insects to feed on plant tissue.
The majority of horticultural oils are made of mineral oil and water, to which an emulsifier has been added. In the past, they contained impurities that could easily burn foliage, but this is rarely a problem with modern oil mixtures.
One of the greatest advantages of oil as an insecticide is that it's effective only in liquid form. Once dried, it's harmless and poses no threat to people or pets. Almost miraculously, it also has little effect on beneficial insects, such as ladybugs and green lacewings, which are able to get out of the way while the oil is applied; by the time they return, it has dried. These oils also prevent many plant viruses that are spread by insects- particularly aphids-and, when diluted and mixed with a small amount of baking soda, they help control powdery mildew.
These formulas are best applied with a hose-end sprayer. Because brands can differ significantly in viscosity and purity, it's essential to follow the manufacturer's directions to the letter. Formerly, dormant (late-winter) oils were often mixed with lime-sulfur compounds to intensify their effectiveness, but I prefer to be cautious and use these ingredients separately, several weeks apart. Products containing sulfur should never be used during the growing season.
For best results when using summer oils, thoroughly water target plants one or two days before applying to ensure turgid foliage. And, because the goal is for the oil to evaporate quickly, spray leaves when humidity is low and no rain is forecast for at least 36 hours. I like to apply the oil early in the morning after the dew is off the foliage so that it's completely dry by mid-morning.