For a long time refined petroleum oils have been used to manage many plant insect pests. The oil smothers the insects by plugging the holes though which they breathe, which are spiracles. The oil may also be toxic to some insects. Mixed in a 1 to 3 percent solution, these oils should be sprayed onto the infested plant parts. There is a chance of damaging plants if the oils are used improperly.
Some plants are especially sensitive to horticultural oils.
Read all labels on the product that you plan to use and make sure you spray the appropriate plants at the correct time. 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.