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
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
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.
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
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.
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.
Two classes of woody ornamentals sensitive to oil when applied under seasonal conditions.
Oil Sensitive Plants
|Plant||Time of Treatment|
|Maples (Japanese, Silver, Sugar)||Dormant|
|Black Walnut, Japanese Walnut||Dormant|
|Blue Spruce/ Koster Spruce||Will lose blue color|
|Alberta Spruce||Late summer|
Plants with a Tendency Toward Sensitivity
|Plant||Time of Treatment|
|Savin Junipers||Summer, Spring|
|Spruce; Norway, White||Dormant|
|Douglas Fir||Dormant, Flowering time|
Table adapted from: Johnson, W.T. 1985. Horticultural Oils, J. of Environmental Horticukure. 3(4): 188 - 191.
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.
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.
Leanne S. Pundt, Associate Extension Educator, Commercial Horticulture
The information in this material is for educational purposes. The recommendations contained are based on the best available knowledge at the time of printingAny reference to commercial products, trade or brand names is for information only, and no endorsement or approval is intended. The Cooperative Extension system does not guarantee or warrant the standard of any products referenced or imply approval of the product to the exclusion of others which also may be available.All agrochemicals/pesticides listed are registered for suggested uses in accordance with federal and Connecticut state laws and regulation as of the date of printing.If the information does not agree with current labeling, follow the label instructions. The label is the law.Warning! Agrochemicals/pesticides are dangerous. Read and follow all instructions and safety precautions on labels. Carefully handle and store agrochemicals/pesticides in originally labeled containers out of the reach of children, pets and livestock. Dispose of empty containers immediately in a safe manner and place. Contact the Connecticut Department of Environmental Protection for current regulations.The user of this information assumes all risks for personal injury or property damage.Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Kirklyn M. Kerr, Director,Cooperative Extension system, University of Connecticut, Storrs. The Connecticut Cooper ooperative Extension System is an equal opportunity employer.