Horticultural oils applied during the dormant season have shown to be effective against overwintering scale insects, mites, mite eggs, and aphid eggs. They are applied when the target insect is most vulnerable, but when the host plant is least exposed.

Horticultural oils kill insects, insect eggs, and mites and mite eggs by contact and only if it is present at the time of application. A pest migrating to a recently sprayed plant will not be affected. Oils kill immatures and adults mainly by suffocation as a result of blocking the spiracles (openings through which insect receive oxygen) resulting in a reduction of oxygen and preventing the exchange of gases for metabolic activity. Death usually occurs within 24 hours. Oils also kill insect eggs by suffocation as well and can penetrate the egg membrane interfering with embryonic development. Oils also may kill beneficial insects, but only if the insect is covered with oil.

When using dormant oils, certain guidelines must be followed in order to insure effective pest control and to prevent plant injury.

• When selecting a spray oil, the unsulfonated residue (UR) rating (level of purity) should be 92% or greater and the distillation (boiling) midpoint should be 412-440°F. (see Table 1)
• Oils should not be applied if temperatures are below 40°F because of freezing and the potential for excessive oil deposition.
• Apply the oil according label rates. Do not overapply.
• Do not spray when buds are fully opened and shoot elongation is occurring.
• Take into consideration plant genetic make-up and variability.
• Finally, and probably most important, be aware and alert to plants that are sensitive to oil sprays. Be sure to read the label for specific plant species and varieties. If in doubt, do not spray! For example, do not spray Colorado blue spruce with oil as it will remove the glaucus (blue) bloom! If you experiment on several plants, be sure to make notes on weather conditions, plant variety and/or species, and application rate. A partial listing of oil sensitive plants and plants with a tendency for sensitivity is shown in Table 2.

Advantages of Horticulture Oils

• Oils are very safe environmentally and are degradable by evaporation.
• Oils quickly dissipate by evaporation shortly after application with little pesticide residue left behind.
• Oils are compatible with an IPM program and are less harmful to nontarget organisms.
• Oils are virtually nonpoisonous to the applicator.
• Oils are noncorrosive to spray equipment.
• Oils can provide a wide range of pest control without plant injury.

Disadvantages of Horticulture Oils ¹

• Oils, if improperly applied during the dormant season, can cause terminal and/or branch dieback which may be apparent in the spring.

THE PLANT HEALTH CARE PROGRAM
THE MORTON ARBORETUM
MARCH 22, 1999

Soybean Aphid Workshop Download and view powerpoint presentations from the workshop....