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Horticultural Spray Oil

Horticultural spray oil has become an essential management tool in the development of integrated pest management programs to control landscape pests. A wide array of oils is available to control insect and mite pests; however, it is essential that you closely examine the label for each individual oil formulation prior to application. Some formulations of horticultural spray oils are labeled for dormant, delayed dormant, and summer application. Likewise, you need to follow all label precautions when any formulation of oil is applied to prevent plant phytotoxicity.

Johnson (1985) defines a horticultural oil as "a highly refined paraffinic petroleum product made solely for use on plants at specific dosages and acts as an insecticide and miticide." Johnson also states that "no horticultural label to date provides enough information to properly identify the substance in the container." Johnson notes that “the minimal information necessary is the U.R. (unsulfonated residue) rating (92 percent) and distillation temperature."

Johnson also states, "The most recent innovation in oil use has been the production of a 412 oil (plus or minus 8°F) that performs best in summer use. It is effective against summer eggs and against a wide range of immature forms, if spray particles strike the insect’s body, i.e., scale crawlers, mealybugs, aphids, leafhoppers, sawfly larvae, naked caterpillars, and beetle larvae."

Plant phytotoxicity remains our biggest concern in the application of horticultural spray oils. The following list summarizes factors that cause phytotoxicity (Johnson 1985).

1. Overdose of oil.
   
   
2.  Wrong timing of spray.
   a. No spraying when buds have fully opened and shoot elongation is occurring.
   b. No spraying when there is an obvious moisture deficit observed in leaves.
   c. No spraying of sensitive plants when the relative humidity is expected to remain over 90 for a period of 48 hours.
   
   
3. Mistaken dormancy in the fall.
   
   
4. Genetic variability.
   

Johnson also lists the following as some oil-sensitive plants.

*Horticultural spray oil labels are not identical. Hence, it is crucial that you follow all label directions. Individual horticultural oil formulations (dormant, delayed dormant, and summer applications) have not been included to control each insect or mite pest because of variations in labels; however, certain formulations can be applied as dormant, delayed dormant, and/or summer treatments. Read each label and apply according to label directions.