Insect and mite pests normally survive the winter months in an overwintering stage such as an egg or a mature female that emerges in the spring. Instead of waiting until spring to initiate control measures, making an application of dormant oil may be beneficial. The advantages of using a dormant oil are (1) a wide range of activity against most species of mites and scales, including activity on eggs; (2) minimal likelihood of insects’ or mites’ developing resistance; (3) a tendency to be less harmful to beneficial insects and predatory mites (= natural enemies) than other pest-control materials with long residual activity; and (4) relatively safe to birds, humans, and other mammals. The disadvantages of using a dormant oil are (1) potential phytotoxicity during the growing season and (2) minimal residual activity or less persistence.
Dormant oils, which are derived from paraffinic crude oil, are the heaviest of the petroleum oil sprays and have a low unsulfonated residue (UR). Unsulfonated residue is a measure of phytotoxic compounds remaining after distillation and refining. A high UR (greater than 92%) indicates a highly refined product with less probability of phytotoxicity. Dormant oils have a UR value below 92%.
Applications of dormant oils are directed primarily at killing overwintering pests, including mites and scales, before they can become active in the spring and cause plant injury. Applications are made in winter to minimize phytotoxicity to plants. Usually a 2 to 4% rate is used in the late fall to early spring. Dormant oils either suffocate, by blocking the breathing pores (spiracles), or penetrate and destroy cells of exposed insects or mites. Oils are contact materials with minimal residual activity once the material has dried, so thorough coverage is essential.
Dormant oil sprays are generally applied to plant parts, which means that the pest’s overwintering stage is located on the plant. However, not all insect and mite pests overwinter on plants. For example, dormant oil applications are not effective on twospotted spider mite, Tetranychus urticae, because the mite overwinters as a female in plant debris, mulch, or other nonplant protected places. In contrast, the spruce spider mite, Oligonychus ununguis, overwinters as an egg on plants, primarily evergreens such as arborvitae, juniper, hemlock, and pine. As a result, this mite is susceptible to dormant oil sprays.
Dormant oils are highly effective in killing the overwintering stages of scales, particularly first and second-instar nymphs (for example, cottony maple scale, Pulvinaria innumerabilis). Euonymus scale, Unaspis euonymi, overwinters as a second instar or mature female and is relatively easy to control with dormant oil sprays. However, scales that overwinter as eggs, such as oystershell scale, Lepidosaphes ulmi, and pine needle scale, Chionaspis pinifoliae, may be more difficult to control. The reason for this is that eggs are generally stacked on top of each other, and the dormant oil may not contact the bottom layer. As a result, additional insecticide applications after egg hatch are generally required.
A concern with the use of dormant oils is phytotoxicity (= plant injury). Some plants, including arborvitae, beech, redbud, and certain maples (Japanese, red, sugar, and amur), are very sensitive to oil sprays. For example, the needles of Colorado blue spruce can be discolored (change from blue to green) from dormant-oil applications. Phytotoxicity is generally prevalent when higher rates (over 4%) are used and when applications are made in early fall before dormancy or in late spring at bud-break. Fewer problems occur when applications are made in late October through February, when plants are completely dormant. To minimize the potential for phytotoxicity, make sure the spray solution is continually agitated.
Dormant oils should never be applied when there is danger of freezing. Dormant oils should be applied to deciduous plants when the ambient temperature will stay above freezing for at least 24 hours. Evergreens are more susceptible to damage, so applications are safe when temperatures stay above 40°F over a 24-hour period. Additionally, dormant oils should never be applied to plants that are stressed, as they are more susceptible to phytotoxicity. Lack of moisture, ex-treme temperatures, sudden change in temperature after spraying, prolonged winds, or poor conditions due to disease or insect infestation predispose plants to phytotoxicity.
It is generally thought that using dormant oils is less likely to result in insect or mite populations’ developing resistance. However, this may not be true. For example, a Christmas tree plantation of Scots pines was sprayed with dormant oils for more than 10 years to control pine needle scale. Eventually the scales became more and more difficult to control. It was discovered that the scale covers were thicker than normal, making it harder for the dormant oil to penetrate.
Preventive dormant oil applications can save time later in dealing with insect or mite pests. Treatments may not be needed in early spring, or the number of applications may be reduced. Reducing the number of insecticide applications preserves natural enemies of mites and scales, including parasitoids and predators, which usually supply sufficient control of these pests.