Most commonly oils are used as dormant sprays to control insects that spend the winter on trees and shrubs. During the growing season, there are refined oils that are effective for controlling whiteflies, young scales, mites and many other familiar plant pests. The one very important point about horticultural oils is that they are quite safe for humans and wildlife. Beneficial insects are not greatly effected by oil sprays, especially dormant oils. Dormant oils are applied when most beneficial insects are not present in the garden.
Oils are like soaps in that they act through contact action and have no residual effects.
These facts make it essential to get thorough coverage when applying an oil to a plant needing protection.