By Ty Torrance

Dormant oils are a good way to control a wide variety of insects.  However, most of the year it is too warm to apply them.  Dormant oils should be applied while plants are dormant or have hardened off for the winter and prior to bud break in the spring.  Applying these oils in high temperatures or before dormancy can cause unacceptable injury, and applying during wet or freezing weather can decrease control.

Most of these oils are combinations of highly refined petroleum oils (or plant derived oils) combined with an emulsifying agent.  They are safe and effective when used properly.  Timing of applications and repeated applications are critical for success.

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 can have different effects on pest insects. The main dormant oil mode of action involves blocking the air holes (spiracles) through which insects breathe, essentially causing them suffocate. In some cases, oils can act as poisons interacting with the fatty acids of insects and interfering with normal metabolism.

Oils pose little risk to humans and most beneficial insects that feed on nuisance species. This allows oils to integrate well with biological controls. Toxicity is minimal compared to alternative pesticides.  Oils quickly dissipate through evaporation, leaving little residue. Oils are easy to apply with standard spray equipment and can be mixed with many other pesticides to enhance their performance.

The main limitation of spraying oils is the small but real potential of causing plant injury or phytotoxicity. Exhibit caution when spraying oils, because many of them have the ability to stain surfaces.  One I have noticed in particular is dark-colored house paints. Some of the newer spray oils do not have this quality if applied properly.

Some of the pests that we can control with dormant oil applications include:  aphids, mites, scale, and any overwintering eggs. In our area, woody ornamental and fruit tree scale insects would be the major pest that we need to focus on.  Soon will be the time to begin apply oils, so you can make a couple of sprays this winter and reduce pest populations for the following spring. 

Please give us a call, if you have any questions (229-248-3033).