Dormant oil sprays are specific, highly-refined petroleum products that are mixed with water and applied to trees and shrubs to control aphids, spider mites, and scale. Dormant oils destroy pests by suffocating them. When applied properly, the thin film of oil plugs the spiracles or pores through which the mite or insect breathes.
Proper timing is critical when using dormant oil sprays. In Iowa, dormant oils should be applied in late March or early April before the plants show signs of breaking dormancy (before "bud break"). Dormant oils applied in late February or early March are not effective as insects are not actively respiring at this time and, therefore, are not vulnerable to the oil's suffocating effects. Dormant oil sprays should be applied as close to bud break as possible.
Dormant oil sprays on fruit trees provide little benefit to most backyard gardeners. Dormant oil sprays do not control the major home orchard insect pests (apple maggot, codling moth caterpillars) and the pests that are controlled by dormant oil sprays are seldom significant (spider mites, aphids).