Horticultural oils have insecticidal activity. They probably work by membrane disruption, not by "suffocation." Professional and amateur horticulturists are often confused about the use of petroleum oils for pest control on ornamental plants. Such oils are called summer oils, dormant oils, and horticultural oils. Horticultural oils and summer oils are synonymous terms. In practice, selecting the proper oil for insect and mite control on ornamental plants is much simpler than it seems. The only formulations for sale in garden shops, nurseries and hardware stores that come in quart sizes or less are horticultural or summer oils.

The confusion arises in unfortunate wording on the labels of many oils that often have directions for winter or dormant applications first and in fairly large type. This leads the buyer to believe the oil is a dormant oil when it is actually a horticultural oil that can be applied during the dormant season also.

There are three factors that distinguish summer oils from dormant oils. These factors determine the effects of the oil sprays on the plant to be treated. Some oils are so toxic to plants that they are actually used as herbicides (Stoddard solvent, diesel oil).

Unsulfonated Residue. Oils have saturated and unsaturated hydrocarbons. Unsaturated hydrocarbons are more unstable than saturated hydrocarbons and when they are sprayed onto plants, they tend to form substances which are toxic to plants. When oil is mixed with strong sulfuric acid, the saturated hydrocarbons react with the acid and sink to the bottom of the mixture as "sulfonated residues". That represents the portion of the oil which would burn leaves and tender stems. The portion left is the "unsulfonated residue" composed of saturated hydrocarbons that are much less likely to burn leaves and tender stems. Dormant oils have 50 to 90% unsulfonated residues (50 to 10% unsaturated hydrocarbons), and they tend to damage green plants and tender stems. Summer oils have 92 to 96% unsulfonated residues (8 to 4% unsaturated hydrocarbons) and they are much safer to use on leaves and stems.

Density. Heavier oils are more toxic to insects than are lighter oils, perhaps because they tend to evaporate slower than lighter oils. Consequently, they are in contact with the target insect for a longer time. However, heavier oils are also more toxic to plants, probably for the same reason. The density of horticultural oils must be balanced by the manufacturers for maximum efficacy and minimal plant injury.

Viscosity. The "body" or "thickness" of oil is measured in arbitrary terms by timing its flow through a standard opening. In general, the faster an oil passes through, the safer it is to use on ornamental plants. However, oils from various parts of the world may have differing viscosities for the same relative plant safety. Nonetheless, viscosity is one of the standards used to characterize insecticidal petroleum oils.

Summer oils are not applied full strength for insect control. They are usually mixed with water at a rate of 1 to 4 parts of oil and 99 to 96 parts of water depending upon the manufacturer's directions for safe use found on the label. There are two types of formulations of summer oils: miscible oils and concentration emulsions. Miscible oils are 95 to 99% oils that form an emulsion immediately when mixed with water. Concentration emulsions are about 80% oil plus emulsifiers and water. Concentration emulsions are thick and resemble mayonnaise or marshmallow topping in appearance.

Summer oils are relatively cheap, but they are also less toxic to insects than many synthetic pesticides and consequently are used at higher rates. Summer oils are used at 1 to 4% of the mixture, whereas some synthetic insecticides are used at a rate of 0.03% of the mixture. An advantage to summer oils is that insects have not developed resistance to oils. Summer oils also have good "spreading" properties and can be used at lower concentrations (2 to 4 teaspoons per gallon) with other synthetic insecticides to enhance the coverage of a pesticide treatment. Because summer oils are safe and easy to mix and because the oils impart a sheen to treated plants, they are popular with homeowners and landscapers for use in controlling ornamental plant pests. Use 3 tablespoons per gallon as a leaf polish. Summer oils have been found to be particularly effective for armored scale control (tea scale, euonymus scale, etc.) and are especially useful in that regard.

Here are some trade names and percentages of various summer oils: Superior Spray Oil (98%, 98.9%), Spray Oil (98%), Scalicide (98%), Summer SpraOil (98%), Superior Oil (98.7a5%), Volck Oil Spray (97%), Volck Supreme Oil Spray (98%), Unico Spray Oil (98%), Oil-I-Cide (80%), All Seasons Hort and Dorm Spray oil.

These names are given to aid the consumer in recognizing summer oils, not as recommendation of any particular products. It pays to shop around for an oil that has many insects on the label and higher rates of application listed. It is very important to read the label of the specific product you choose to use.

Rates of Various Oils Labeled for Each Pest: The summer rate per gallon of water is given in bold (and the dormant season rate per gallon of water is given in plain text in parentheses).

Oils are among the safest insecticides in use and are effective for a variety of soft bodied insects such as aphids, scale, and whiteflies that will be in direct contact with an oil spray. Resistance should generally not be expected to develop with oils and they tend to have among the lowest nontarget effects. Be sure to heed all warnings about applying while the plant is in direct sunlight and during temperature extremes.

Nonsynthetic based oils are appearing in the market. These are products produced from sources such as fish oil, sesame seed (Organocide), neem seed (Trilogy), rosemary (Ecotrol), and soy oil and in combinations. In some cases, but not all, they have performed well. Phytotoxic effects are still possible even with these organic oil products.

• Biopesticides: Horticultural Oils, Natural pest control with oils. Anonymous. 2022. Inst. of Food & Agr. Sci. (IFAS), University of Florida.
• Horticultural Oils, IPM Spotlight: Horticultural Oils. Corser, J. S. 2013. Durham Extension Master Gardeners. NC State Extension.
• Horticultural Oils – What a Gardener Needs to Know. J. Skelly. 2013. University of Nevada Cooperative Extension Fact Sheet 13-20.
• Insect Control: Horticultural Oils . Cranshaw, W. S. 2013. Colorado State University Extension fact sheet – 5.569.

• North Carolina Agricultural Chemicals Manual
• More information on insects from NC State Extension
• NC State Extension Plant Pathology Publications and Factsheets
• NC State Extension Horticultural Science Publications
• Find your local N.C. Cooperative Extension center

Recommendations of specific chemicals are based upon information on the manufacturer's label and performance in a limited number of trials. Because environmental conditions and methods of application by growers may vary widely, performance of the chemical will not always conform to the safety and pest control standards indicated by experimental data.