Dormant oil sprays are effective against many overwintering insect and mite pests. These sprays are highly refined petroleum oils (paraffinic oils) that are applied when trees and shrubs are dormant. Deciduous trees and shrubs are considered to be dormant from the time that the leaves fall in autumn until bud break in spring. Evergreens are considered to be dormant during that same time period.
Because the plant is dormant, this oil is not as likely to cause damage as during the growing season. Even so, application is recommended to deciduous ornamentals when the temperature will remain above freezing for 24 hours after application. For evergreens, it is recommended that the temperature remain above 40°F for 24 hours after application. In recent years, these oils have become more highly refined, reducing the likelihood of damage. Even so, following these guidelines of application results in the more rapid evaporation of the oil, providing an extra measure of safety.
Dormant oil sprays are effective against many aphid, mite, and scale, killing the overwintering nymph, adult, or exposed eggs. Dormant oil is not effective against scale that overwinter as eggs, including oystershell scale and pine needle scale. It is also not effective against twospotted spider mite because it overwinters beneath the tree.
Oil spray is sold as Sunspray, Volck Oil Spray, Horticultural Oil, and other names. The label typically has a summer oil spray rate and a higher, dormant oil spray rate. Oil sprays are very phytotoxic to Japanese and sugar maple, as well as some other plants. Be sure to check for warnings on the label.