What They Are and How To Use Them By Dr. Heidi Kratsch and Wendy Hanson Mazet, University of Nevada Cooperative Extension

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orticultural oils are products made from plant- or petroleum-based oils that are diluted with water and applied as a spray to control a variety of landscape pests. They are effective on soft-bodied insects, arachnids (mites) and some fungal diseases, especially powdery mildew. Oils are commonly used as effective, safe alternatives to synthetic pesticides because they decompose quickly in the environment and pose minimum risk to humans, pets, wildlife or natural enemies. Horticultural oils used as insecticides kill insects by creating a physical barrier to respiration, clogging the breathing pores along the sides of adult insects and larvae. They can also be used on insect egg masses to limit oxygen uptake, disrupting egg hatch; and they can disrupt insect feeding on oil-covered surfaces. Horticultural oils used as fungicides inhibit fungal growth and reduce spore germination. All horticultural oils must come in contact with the pest to be effective. Horticultural oils go by many names that can be as confusing as plant common names when it comes to figuring out how to use them. We’ll describe some common types of oils here, but keep in mind that you must read the label for use directions. Not only is it federal law, but the time of year and the concentration you use depend on what pest you are trying to manage. Petroleum-based horticultural oils can be categorized by their unsulfonated residue (UR) percentage, which is a measure of the degree of refinement of the oil. The higher the UR percentage, the lower the saturated hydrocarbon content of the oil and the lower the risk of phytotoxicity (plant damage) to leaves:

DORMANT OIL – used on woody plants during the dormant season to target overwintering insects and their eggs. Dormant oils may have a UR between 50 and 90 percent. Summer oil – used on plants when leaves are present. Summer oils typically have a UR between 92 and 96 percent. STYLET OIL – used to manage insect-vectored plant viruses. Stylet oils have a UR above 99 percent. The more highly refined summer oils are further categorized by the distillation temperature range of the product. This range represents the boiling point of the oil at 10 and 90 percent distillation. Generally, the narrower the temperature range, the more predictable the performance of the oil when sprayed on plants.

according to label directions. They distill over a narrow range of temperatures. NARROW-RANGE OIL – highly refined oil with a narrow range of distillation. Narrow-range oils are a class of superior oil, and often the terms are used interchangeably. SUPREME OIL – category of highly refined oils that distill at slightly higher temperatures and over a wider range than narrow-range oils. Most supreme oils can be used year-round. Plant-based oils, derived from plant seeds, leaves, stem or flowers, are common in organic systems. Soybean oil, cottonseed oil and canola oil are used, as well as neem oil. Neem oil is derived from the neem tree. The oil fraction of neem seed and other plant-based extracts are used the same way as petroleum-based oils. They are formulated with an emulsifier to permit mixing with water and diluted to concentrations similar to petroleum-based oils (0.5 to 2.0 percent). They are effective against insects and fungal diseases. Some neem products, however, contain the active ingredient azadirachtin, extracted from the neem seed. Products containing azadirachtin have a different mode of action than horticultural oils and are actually categorized as botanical insecticides. They are used as an insect feeding deterrent and have growth regulator activity against insects. Your choice of horticultural oil is dictated by the life history of the pest you are trying to manage and the challenges that exist in achieving good control. If the vulnerable stage of the insect’s development is during its overwintering period, a dormant application would be best. Insects that show up in spring or summer are best controlled with summer-type oils. However, some products are labeled as both “all-season” and “dormant” oil spray. The names refer to the time of year the oil can be used, with higher concentrations (up to 4 percent) recommended when the product is used as a dormant spray. Always follow label instructions for proper rate of application. Finally, horticultural oils are safe and effective when used as directed, but some precautions apply. Do not apply when temperatures are above 85 degrees Fahrenheit or below freezing. Avoid application to quickly expanding shoots or as a dormant application in fall before plants have hardened off. Make sure to agitate the oil mixture regularly during application to prevent separation. Avoid spraying oils when pollinators or other beneficial insects are present.

SUPERIOR OILS – products that have reached a level of purification that allows for year-round use without phytotoxicity, if used

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