Insect Control: Horticultural Oils: Quick Facts

We want to thank the Colorado State University Extension , more particularly W.S. Cranshaw and B. Baxendale for putting together this report. W.S. Cranshaw, is an entomologist and professor, bioagricultural sciences and pest management at Colorado State University Extension; and B. Baxendale, is a professor in botany at Teikyo Loretto Heights University. Our next tips will be extracts from their report.


Quick Facts.:
  • Certain oils, diluted with water and applied as sprays, can be effective controls of many plant pests.
  • Horticultural oils are usually highly refined petroleum oils combined with an emulsifiying agent. Some plant-derived oils also are used.
  • Advantages of oils include safety, effectiveness and limited effects on beneficial insects.
  • Do not use oils on certain sensitive plants. Plants under drought stress may have increased risk of injury.
Various oils have been used for centuries to control insect and mite pests. Oils remain an important tool to manage certain pest problems (e.g., scales, aphids, mites) on fruit trees, shade trees and woody ornamental plants. Several recently developed oils extend this usefulness to flowers, vegetables and other herbaceous plants. Oils also can control some plant diseases, such as powdery mildew. Oils used to protect plants have been called by many names, but perhaps horticultural oils best describes them.
Oils have different effects on pest insects. The most important is that they block the air holes (spiracles) through which insects breathe, causing them to die from asphyxiation. In some cases, oils also may act as poisons, interacting with the fatty acids of the insect and interfering with normal metabolism. Oils also may disrupt how an insect feeds, a feature that is particularly important in the transmission of some plant viruses by aphids.
Oils pose few risks to people or to most desirable species, including beneficial natural enemies of insect pests. This allows oils to integrate well with biological controls. Toxicity is minimal, at least compared to alternative pesticides, and oils quickly dissipate through evaporation, leaving little residue. Oils also are easy to apply with existing spray equipment and can be mixed with many other pesticides to extend their performance.
The main limitation of spray oils is their small but real potential to cause plant injury (phytotoxicity) in some situations. Oils also can stain some surfaces, particularly dark-colored house paints. Some of the newer spray oils can largely eliminate these problems if they are properly applied.

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