As long as humans have been raising plants for food, they have been trying to control pests. Sulfur compounds were applied as far back as 2500 BC. In 350 BC, Romans used oil and ash for pest control. The Chinese used soap to control insects in 1100 AD. By the 1880s in the United States, horticultural oils were used regularly as dormant sprays on fruit trees. Horticultural oils continue to be applied today for pest control.
Horticultural oils are pesticides that control insects, mites and some plant diseases. They are specifically designed to control plant pests. Commercially available horticultural oils are highly refined petroleum products that are filtered and distilled to remove compounds that can harm plants. In addition to petroleum-based products, there are plant-based horticultural oils available. These may contain soybean, cottonseed, sesame, neem or other oils. Plant-based horticultural oils are less refined and may burn plants more readily than petroleum-based products.
Historically, horticultural oils were called dormant oils because they were sprayed only when plants, particularly fruit and shade trees, were in a dormant stage of growth before buds opened in the spring. Oils are most effective against exposed eggs, immature stages and soft-bodied adult insects. These include scales, aphids, whiteflies, mealybugs, thrips, leafhoppers and arachnids, such as spider mites, on fruit or shade trees and on many ornamental plants. In the winter, dormant sprays only kill overwintering insects and exposed eggs.
Usually, a higher rate of oil is mixed and applied during dormant or delayed dormant than with a summer application. The proper rate is listed on the label. Enough water must be mixed with the oil to cover all the bark cracks and crevices. According to University of Nebraska-Lincoln Extension, a 20-foot tree will probably require four gallons of water for complete coverage.
Dormant oil should not be sprayed 48 hours before or after a freeze occurs or is predicted.
Always read and follow all 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.
Horticultural oils work well to control insect pests and, if used properly, can be a less-toxic approach than chemical-based insecticides. A dormant or delayed dormant application can kill many overwintering insect pests that would normally plague plants in the late spring, such as aphids. Treating in the winter or early spring can save time and avoid later plant problems.
JoAnne Skelly is the Carson City/Storey County Extension educator for the University of Nevada Cooperative Extension.