Various oils have been used to control insect and mite pests for hundreds of years. Recently, renewed attention has focused on the use of oils as a "natural" substitute for traditional insecticides.
What used to be called "dormant oil" is now properly referred to as "horticultural oil". The name "dormant" referred to the stage of plant development when the oil spray was applied. Today's horticultural oils have wide range applications extending the entire season when properly used.
The horticultural oils are used to control overwintering eggs of red spiders, scale insects, aphids, bud moths, leaf roller, red bug, codling moth, blister mites, galls, whitefly, mealy bugs and others. Horticultural oils are highly recommended for use on fruit trees, shade trees, shrubs, and ornamentals where these pests are common.
Advantages: Oil sprays are exceptionally safe to humans. They have little if any effect on wildlife and non-target insects in the environment (ladybugs, parasitic wasps, honey bees etc.) Oil sprays are less toxic because of the method by which they kill target pests. The thin film of oil covers the target insect or mite and plugs the spiracles or pores through which it breathes. The cause of death is primarily suffocation. Large insects and animales that breathe by another method are not affected by these oils.
Another advantage of oil applications is the absence of objectionable odors.
Disadvantages: The major limitation is the oil is only effective against those pests that are thoroughly coated by the spray solution. This usually means small, immobile or slow moving pests that are exposed on the surface of the plant at the time of application will be controlled.
Since oil sprays only work by contacting and covering the target pest, thorough application is essential.
If you use oil sprays, read and follow all label directions. Oils must be mixed exactly at the right dilution rate to prevent plant damage. This is especially true for summer applications.
Timing: Proper timing is critical for success when using oils, Dormant applications should be applied in late March or April before leaves or flowers show signs of breaking dormancy. Wait until as close to bud brask as possible before applying oil sprays. For summer use, oils are effective against insects that are "soft and slow".