Horticultural Oils

Until recently, horticultural spray oils were primarily used as "dormant oils" applied before bud break in the spring. A new generation of "superior horticultural oils or summer oils" can be safely used on many woody ornamental and bedding plants during the growing season.

Horticultural oils are applied as sprays and function by direct contact with the insect control. However, the primary mode of action is through suffocation. This results when the spray forms a coating of oil on the insect's body, blocking the spiracles or breathing opening. Horticultural oils also may interact with insect fatty acids disrupting cell membranes and metabolism. Horticultural oils may have repellent properties for some insects. For example, adult greenhouse whiteflies quickly leave poinsettia plants that are treated with horticultural oil. In one study, 95% fewer eggs were laid on plants treated with oil compared with poinsettias not treated with oil. Horticultural oil may act as an antifeedant (discouraging feeding) for the euonymus webworm and some leafrollers.

Superior horticultural oils are made from a complex mixture of petroleum hydrocarbons produced from paraffinic crude oil. As such, oils cannot be identified by a specific chemical formula as are the more "traditional" pesticides. Factors such as the unsulfonated residue (u.r. rating) and distillation temperature, which are listed on the label, provide helpful information on the oil's key characteristics.

The u.r. rating is the percentage of saturated compounds in the oil and represents the oil's purity of degree of refinement. Horticultural oils are from 92 to 99 percent pure.

The distillation temperature, or the range over which an oil is distilled, is a measure of the oil's volatility. Horticultural oils may have distillation temperatures from 414 Fahrenheit to 468Fahrenheit. Lower distillation temperatures (i.e., 414 Fahrenheit), represent oils that are more volatile and may be used in the summer or dormant season. For these more volatile oils, the dilution rate may be adjusted from 3% for dormant applications to 2% for summer applications. When the distillation temperature is higher (i.e., 468 Fahrenheit), the oil is less volatile and should be used for dormant applications only.

Benefits of Horticultural Oils

Horticultural oils are most effective against many soft bodied pests including aphids, adelgids, spider mites, scale insects, greenhouse whiteflies, mealybugs, psyllids, plant bugs, lace bugs and some caterpillars. Recent research shows promise for horticultural oil's future role as a fungicide against powdery mildew on lilac, rose, horse chestnut and other plant species.

Such oils are also safe to mammals (including humans), some birds and reptiles but are toxic to fish. Horticultural oil is not selective and, therefore, will kill any susceptible beneficial insects, as well as pests, that are coated by the oil. For example, when treating for spider mites, predatory mite eggs will be killed by the oil as easily as spider mite eggs. However, horticultural oil evaporates rapidly, and, when dried, it will have little toxic activity. Thus, beneficial insects may safely reenter oil treated areas after the spray residue has dried. Spot treatments with oil will minimize the impact on beneficial insects, because only those individuals covered by the oil will be affected. During one hundred years of use, researchers have recorded only one example of resistance with horticulture oil.

Limitations of Horticultural Oils

In spite of these benefits, many potential users do not use horticultural oils because of a concern that phytotoxicity (toxicity to plants) will occur. The heavier, less-purified dormant oils used in the past, were more likely to cause phytotoxic reactions than the more highly refined oils in use today. Superior horticultural oils can now be safely used on more than forty-five different types of woody ornamentals. Many bedding plants including ageratum, marigolds, salvia, Vinca rosea, and others may also be safely treated with horticultural oil. Treating plants that are under moisture or drought stress during summer may result in marginal leaf burns. Oil sprays also should be avoided when the relative humidity is high (over 90% for 48 hours), because the slower drying time of the oil on the foliage may cause phytotoxicity. During summer, leave at least a two-week interval between treatments. Also avoid dormant oil treatments in the fall, before true dormancy has taken place, or twig and shoot dieback on deciduous plants may occur the following spring. Phytotoxic symptoms may first appear as a general chlorosis as the foliage turns a light yellow. Treated leaves may appear water-soaked, turn dark purple and drop from the plant. Treating bedding plants in flower may result in spotting of the flowers. Oil sprays on wax begonia and coleus may cause pitting and speckling of the foliage.

Treating blue spruce or blue rug junipers, results in a temporary loss of blue color, although they will not be damaged permanently. If uncertain of the reaction of a particular cultivar, spot treat with oil before infestations occur. This will determine if horticultural oil can be safely used should pest problems develop later.

Horticultural oils are an effective low toxicity alternative for managing pests when they are used properly with an understanding of their benefits and limitations.

References

Baxendale, R.W. and W.T. Johnson. 1988. An Evaluation of Dormant Oil Phylotoxicity on Six Species of Woody Ornamentals. J. of Arboriculture. 14(4): 102-105.

Baxendale, R.W. and W.T. Johnson. 1988a. Evaluation of Summer Oil Spray on Amenity Plants. J. of Arboriculture. 14(9): 220-225.

Baxendale, R.W. and W.T. Johnson. 1989. Update Note Concerning Horticultural Oil Concentrations for Verdant Use. J.. of Arboriculture. 15(2):51-52.

Gill, S. and W. Healy. 1990. Spray Oils May be Your Best Defense in an IPM Program. GrowerTaiks. 53 (11):68-69.

Grossman. J. 1990. Horticultural Oils: New Summer Uses on Ornamental Plant Pests. The IPM Practioner. 12(8)1-10.

Johnson, W.T. 1985. Horticultural Oils. J. of Environmental Horticulture. 3(4): 188-191.

Johnson, W. T. 1991. Rediscovering Horticultural Oils. American Nurseryman. 173 (1): 77-80.

Larew, H.G. and J.C. Locke. 1990. Repellency and Toxicity of a Horticultural Oil against Whiteflies on Chrysanthemum. HortScience. 35(11) 1406-1407.

Prepared by:
Leanne S. Pundt, Associate Extension Educator, Commercial Horticulture

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