Horticultural oils are particularly useful in the organic orchard where they control pests that overwinter in bark crevices.
This article is part of our Organic Pest Control Series, which includes articles on attracting beneficial insects, controlling specific garden pests, and using organic pesticides.
When applied directly to pests, horticultural oils interfere with respiration, causing insects to suffocate and die. These oils can also kill beneficial mites and cause leaf injury to some plants, and frequent use can reduce yields, even when the pest is controlled. Best applied in cool weather, horticultural oils are valuable tools in the organic orchard, where they can be used to control pests that overwinter in bark crevices. Oily leaf surfaces also make poor sites for insects to lay eggs, and may deter early outbreaks of mites, aphids and scale on fruit trees.
Some horticultural oils include herbal essential oils, which may repel some pests and suppress some diseases. Neem oil is the only horticultural oil considered to be an active ingredient in pesticides.
Highly refined mineral oils, often called superior oils, evaporate quickly so they are less likely to injure foliage compared to heavier oils. Superior oils often control powdery mildew in addition to spider mites, whiteflies and other difficult pests. Only a few products including JMS Stylet Oil and Pure Spray Green are approved for use in the organic production of fruits and vegetables.
Insects that overwinter in bark crevices such as aphids, mites, mealybugs, and scale, as well as eggs of some caterpillars, are killed by early spring applications to fruit trees. In the vegetable garden, spider mites on cucumber family crops may be brought under control with horticultural oil. Oils also are used to prevent powdery mildew and black spot on roses.
The recommended practice is to spray trees in spring, just before flower buds open. Do this during a period of mild weather when temperatures are expected to stay above 40 degrees Fahrenheit. Never apply horticultural oil in very warm weather (above 80 degrees) because heat increases risk of leaf injury. Also, do not apply on humid evenings, because slow drying of leaf surfaces increases risk of leaf injury. Always follow label directions for diluting concentrated products.
Mix only as much concentrate as you will need. If not used within a few days, dispose of unused solution by diluting it with water and pouring it out in a sunny spot, away from water supplies. Store horticultural oils in their original containers on a high shelf, out of the reach of children or pets, in a cool place where temperatures will not exceed 100 degrees. Under good storage conditions, horticultural oils may last five years.
More information on horticultural oils is available from Colorado State University.