Horticultural oils, which are relatively safe for humans and wildlife, are pesticides that control insects, mites and some plant diseases. Commercially available horticultural oils are highly refined petroleum products that are distilled to remove compounds that can harm plants. They are 92 percent to 99 percent pure.
After distillation, they are then formulated with a mixing agent to blend with water for ease of application. In addition to petroleum-based products, plant-based horticultural oils are also available. These may contain soybean, cottonseed, sesame, neem or other oils. However, plant-based horticultural oils are less refined and may burn plants more readily.
Oils are most effective against exposed eggs, immature stages and soft-bodied adult insects. These include scales, aphids, whiteflies, mealybugs, thrips, and leafhoppers, as well as arachnids such as spider mites, on fruit or shade trees and on many ornamental plants. Oils are also used to control diseases such as powdery mildew. And, by killing the insects that spread viruses, viral diseases can be reduced.
In the winter, dormant oils only kill overwintering insects and exposed eggs. At any season of application, oils control insects with direct contact only. Insects must be present for the oil to work, because oils have no effect after they have dried. When a horticultural oil is sprayed onto a plant, it covers any exposed insects or eggs and suffocates them. In addition, the oil may disrupt how an insect feeds or may interfere with its cell membranes or metabolism. Complete coverage of the insect population is required for a treatment to be effective. This is a nonselective process, with the oil killing almost any insect, pest or beneficial, as well as reducing hatching success in eggs.
There are two methods for dormant horticultural oil application. One is to apply it before the buds break or show any color. However, this can speed up spring bud development, making buds more susceptible to frost and cold temperature damage. The second method is the delayed dormant application. This occurs when buds are open at the tip.
Dormant oil should not be sprayed 48 hours before a freeze is predicted or 48 hours after one occurs. Usually, a higher rate of oil is applied during dormant or delayed dormant than with a summer application. The proper rate is listed on the label.
Always read and follow label directions for proper timing and rates dependent on the stage of the life cycle of the pest. Apply only when the pest is present.
Sufficient water must be mixed with the oil to cover all the bark cracks and crevices.
For information, see my publication “Horticultural Oils, What a Gardener Needs to Know” at https://extension.unr.edu/publication.aspx?PubID=3029.
JoAnne Skelly is associate professor & extension educator emerita for the University of Nevada Cooperative Extension. Email skellyj@unr.edu.
