Various types of oils have been used for many centuries to control hard-to-kill insects like aphids, scale and spidermites on fruit trees, providing good control of these insects with little risk to the plant, beneficial insects or the applicator when used according to label directions. Oils kill insects in several ways, most importantly by blocking the air holes through which the insects breathe, causing them to suffocate. Oils may also acts as poisons by interfering with insect metabolism or disrupt insect feeding. Oils act like a contact insecticide and provide no residual control, so the insects must be present and in a vulnerable stage of development for an oil application to be effective. The development of insect resistance to oils is rare, since the mode of action is mechanical rather than chemical.

They are also effective against powdery mildew and some aphid-transmitted viruses. In fact, oils can easily be worked into an organic spray program for fruit trees as well as small fruit plants, vegetables, flowers, tree and shrubs.

Terms
Some confusing terminology has arisen regarding horticultural oils, including the following:

• Horticultural oils are products used to control pests on plants.
• Dormant oil is a term that was originally used to refer to heavy grade oil products used on woody plants during the dormant season. These older products caused phytotoxic effects if used during the growing season, so were limited to dormant season applications. These products have now been replaced in the horticultural market with lighter grade, more highly refined products that can be used both in winter and summer. The term dormant oil now refers to the time of application rather than the product. Dormant oil applications target overwintering insects and can kill adults, nymphs and eggs of insects like mites.
• Summer oil is a term that refers to lighter grade oil products that can be used when plants are actively growing and in full leaf. The leaf damaging components like sulfur have been largely removed, therefore summer oils can be used in both winter and summer. The term is now more commonly used to refer to the time of application rather than the product used.
• Superior oil refers to oil products that have reached a level of purification that allows year-round use without phytotoxicity if used according to label directions. These products distill over a narrow range of temperatures.
• Supreme oil is a term used to categorize highly refined oils that distill at slightly higher temperatures. Most supreme oils meet the characteristics of a superior oil.
• Some common brand names of commercially available oil products include Sunspray ®, Ultra-Fine and Volck ®. These products are superior oils and can be used both in summer and during the dormant season.

Horticultural oils have an emulsifier component added, allowing them to mix with water. Other types of oil, including vegetable oil or automotive oil, do not contain emulsifiers so will not mix with water; consequently, these products should not be used on plants and may cause plant damage.

History of Dormant Oil Applications
Before the development of current oil refining techniques, oils were used primarily on fruit trees during the dormant season, before the spring bud break. The application was timed to kill insects overwintering on the trees but also to avoid burning or damaging the leaves, which frequently happened with heavier, less highly refined oils. These oil products could not be used on plants during the growing season due to their phytotoxic effects. For this reason, the term 'dormant oil' was coined. Oil products currently available are more highly refined and the leaf damaging components including sulfur have been largely removed. Products like Sunspray® , Volck® and superior oil can be used both in summer and during the dormant season. Summer foliage applications of horticultural or 'summer oils' can control such insects as aphids, leafhoppers, whiteflies, scale and spidermites.

Dormant Season Fruit Tree Applications
Fruit tree oil applications are commonly recommended as either a dormant or delayed dormant application.   Dormant oil applications darken tree bark and buds, speeding up spring bud development. This reduces the flower buds ability to withstand cold temperatures, therefore, applications are best delayed until just before bud break.

• Dormant application- A dormant application is made just before bud break and is targeted to control Eriophyid mites, San Jose and oystershell scale.
• Delayed dormant application- A delayed-dormant application is made when plants are in the green tip (fruit buds are open at tip showing 1/16 inch of green) through half-inch green bud growth stage (1/2 inch of leaf tissue is projecting from the fruit buds) for fruit trees to control insects such as San Jose scale, European red mite eggs, and aphid eggs.

Many labels will recommend higher rates of oil be used for a dormant application and slightly lower rates for a delayed-dormant application, however, always follow label directions and rate information when mixing oil products.

Dormant and delayed-dormant oil applications are most effective when applied with large quantities of water, which is necessary to thoroughly cover all the bark cracks and crevices for mite and scale control. Also make sure branches in the center of the tree receive adequate coverage. Poorly pruned trees with heavy center branching will required higher amounts of water for thorough coverage.  Based on tree height, a 20 feet tall tree should be sprayed using a minimum of 4 gallons of water; tree height 15 feet, 3 gallons; tree height 10 feet, 2 gallons; and tree height 5 feet, 1 gallon.

Later season fruit tree applications usually are made using an orchard spray, which incorporates both a fungicide and insecticide for control of both disease and insect problems. 

Insects Controlled with Horticultural Oils
Insects such as adelgids, aphids, leafhoppers, scale insects, mites and whiteflies can be controlled on landscape ornamentals with oil applications.

• Aphids- many hosts. Spray affected plants during the growing season when mites are detected.
• Oystershell scale- many hosts. Spray affected plants after eggs hatch and light yellow crawlers appear (late May). Repeat in 7-10 days, unless prohibited by the label. Oil may damage some maples.
• Pine needle scale- hosts include pine and spruce trees. Thoroughly spray foliage, branches and trunk with horticultural oil in March.
• Spruce spider mite- hosts include spruce, juniper and pine. Spray affected trees during the dormant season just before bud break. (April) Repeat applications when mites are detected during the growing season (April to May, and September to October). Make the first application then a second application 7-10 days later. Oil will remove the blue needle color of spruce.
• Twospotted spider mite- many hosts. Spray affected trees during the growing season when mites are detected. Make the first application then a second application 7-10 days later. Oil will remove the blue needle color of spruce.
• Whiteflies- many hosts. Only nymphs are killed and thorough coverage of the undersides of leaves is necessary for control. Spray affected trees during the growing season when whiteflies are detected. Repeat applications as needed and according to label directions.

To get the best results from a horticultural oil application, follow these precautions.

• First, read and follow all label directions, paying particular attention to which plants are listed on the label as sensitive to the product.
• Do not apply oils to sensitive plants or allow the product to drift onto them.
• Do not combine oil applications with sulfur or pesticides containing sulfur like Captan.
• Do not apply horticultural oil when temperatures are above 100 degrees or below freezing.
• Do not apply oils to quickly expanding shoots in spring.
• Finally, allow plants to harden off well in fall before making dormant oil applications; this avoids increased susceptibility to winterkill.