This oily shroud on tree and shrub branches suffocates eggs and the soft bodies of insect pests such as aphids, mites, scales, and mealybugs.
Dormant oil is relatively safe to other animals, including beneficial insects, birds, and humans.
Dormant oil is not a new pesticide. Two thousand years ago, Pliny the Elder mentioned using it to control plant pests. Oils can damage plants, however, so they should be used with special precautions. A few decades ago, an oil spray that is less damaging to plants was developed. Summer oil can be sprayed in cold weather, as long as the temperature is not so cold that the oil freezes, or in hot weather, as long as the plant is not under water stress.