Horticultural oils are among the least toxic pesticides available to control fruit tree and shade tree pests and diseases. They are considered safe and effective pesticides that decompose quickly in the environment and pose little risk to humans, pets, wildlife and beneficial insects.
Horticultural oils are refined plant- or petroleum-based oils that are mixed with water and applied as a spray to control a variety of landscape pests. They are effective in controlling soft-bodied insects, such as scales, aphids, whiteflies, mealybugs, thrips, leafhoppers and arachnids such as spider mites. Controlling these pests helps control the viral diseases some of them can spread. Some formulations are also effective fungicides, helping to control diseases such as powdery mildew, downy mildew, rust and leaf spot.
Horticultural oils go by many names and have many formulations. As with all pesticides, you must read, understand and follow the label directions. Not only is this the law, but the formulations and concentrations of the oil applied depend on the time of year, the growth stage of your tree or shrub, the temperature, and the pest you are trying to control. Dormant oils are those normally applied when the tree or shrub is dormant, generally in the winter. These oils are not as refined and can cause damage to leaves. Summer oils are horticultural oils that are more refined and are less likely to cause damage to leaves. They are also referred to as Superior oils or Supreme oils. Some oils are labeled as both all-season and dormant oil spray. By varying the concentration, these oils can be used year round.
How do horticultural oils work? Insects are controlled by direct contact with the oil, so the insect or its eggs must be present for the oil to work. The oils coat and smother insect pests and their eggs. Horticultural oils have no residual effect after the application, so once the oil dries it will no longer control insect pests. Horticultural oils should not be sprayed when the temperatures are below freezing or above 85 degrees Fahrenheit. Horticultural oils are not selective, that is they can kill unintended beneficial insects, including bees. Use caution and try to spray in early morning or late evening when these beneficial insects are less active.
Your choice of horticultural oil is dictated by the pest you are trying to control, the pest’s life cycle and the plant that is affected by the pest. Most insects have stages in their life cycle when they are most susceptible to control; you should time your application to coincide with these susceptible stages. Horticultural oils control fungi by inhibiting fungal growth and reducing fungal spore germination. The fungi must be present for the horticultural oil to control. Horticultural oils will not prevent a fungal disease. Want more information? Check out the Cooperative Extension fact sheet Horticultural Oils – What a Gardener Needs to Know at http://www.unce.unr.edu/publications/files/ho/2013/fs1320.pdf.
Melody Hefner is the Urban IPM and Pesticide Safety Program Coordinator for the University of Nevada Cooperative Extension. Have questions about controlling pests in your landscape? Contact a master gardener at firstname.lastname@example.org.