Dormant oils are horticultural oils used to control overwintering insects, such as aphids and mites, that live in the bark of fruit trees. They are applied by pump sprayers or compressed air sprayers, usually after pruning in late winter to early spring. Dormant oil sprays treat specific problems and aren't all-purpose insecticides. Pests should be properly identified to make sure a spraying is warranted.
Dormant oil sprays are sprays made from oils derived from petroleum oil or occasionally cottonseed oil, according to the University of Illinois. They contain emulsifiers, allowing them to mix with water, and may contain insecticides, lime-sulfur or copper.
Dormant oils effectively control overwintering pests, including scale, spider mite eggs, blister mites, bud mites, rust mites, aphid eggs, fruit tree leaf roller eggs, cutworms, and twig borers, according to Utah County Extension. They are relatively safe for humans, pets and wildlife, and rarely kill beneficial insects.
Dormant oils don't destroy apple maggot, codling moth, apple scab or brown rot, according to Washington State University. They also are not effective once dry, so they don't prevent future infestations. They may damage fruit trees if sprayed after buds have opened or during freezing temperatures.
Dormant oils are sprayed on trees while they are in a dormant state, mid-winter to early spring. They are also used occasionally for fall spraying to treat bacterial and fungal diseases, including leaf curl, shot hose and powdery mildew, according to the University of California Davis. Copper is added to dormant oils when used to treat disease. They should not be used during rainy or foggy weather or when temperatures are below freezing.
Dormant oil won't treat every insect infestation and should be considered part of an overall pest management program, according to Washington State University. Spraying trees after flowers have bloomed kills bees and can damage the blossoms.