Dormant oil sprays are an effective way to manage scale insect problems. Application can be made during warm spells in the winter to greatly reduce problems during the following growing season. Of the common scale insects in Illinois, only oystershell scale and pine needle scale are not effectively controlled with dormant oil. Oystershell scale overwinters in the egg stage, and much of the pine needle scale population overwinters as eggs. Scale eggs are not controlled effectively with oil.
Applications of dormant oil can be made as long as the plant is dormant. With deciduous trees and shrubs, dormancy is between leaf drop in the fall and bud break in the spring. For some oaks, other trees that dont drop their leaves in the fall, broadleaf evergreens, and conifers, one can assume that they are dormant during the same time period that most deciduous trees and shrubs are leafless.
Apply dormant oil on days when the temperature will stay above freezing for 24 hours after application. This allows the oil to evaporate off of the tree and not soak into the plant where it may cause phytotoxicity. Because evergreens, both broadleaved and conifers, have more relative surface area, spray them only on days where the temperature will stay above 40°F. for 24 hours after application.
Some petroleum spray oils will be marketed as dormant oils. Those labeled as horticultural oil, spray oil, and other names will commonly have different rates for summer oil sprays versus dormant oil sprays. Typically, dormant oil sprays are mixed to be applied with a concentration of 2 to 4% oil. It is recommended to apply the lower rate to evergreens, using the higher rates on deciduous plants. See the label for directions on mixing and application. Petroleum oils will remove the blue bloom on Colorado blue spruce and Koster spruce and can be phytotoxic to Japanese maple, walnut, arborvitae, Cryptomeria, Japanese holly, red cedar, and smoke tree.