Insect and mite pests normally survive the winter months in an overwintering stage, perhaps as an egg or a mature female that emerges in the spring. Instead of waiting until spring to initiate control measures, it is possible to apply a dormant oil. Advantages include (1) a wide range of activity against most species of mites and scales, including effectiveness on eggs; (2) little 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) relative safety to birds, humans, and other mammals. Disadvantages are (1) potential phytotoxicity during the growing season and (2) minimal resid-ual activity or less persistence.
Dormant oils, derived from paraffinic crude oil, are the heaviest of the petroleum oil sprays and have a low unsulfonated residue (UR). UR 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%.
Dormant oil applications 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 win-ter to minimize phytotoxicity to plants. Usually, a 2% to 4% rate is used in the late fall to early spring. Dormant oils suffocate insects or mites by blocking their breathing pores (spiracles). These oils are contact materials with minimal residual activity, 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 will not work on two-spotted 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, especially first- and second-instar nymphs (for example, cottony maple scale, Pulvinaria innumerabilis). Similarly, euonymus scale, Unaspis euonymi, overwinters as second instars or mature females 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 hard to control. Eggs are generally stacked on top of each other, and the dormant oil may not contact the bottom layer. 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, and certain maples, are very sensitive to these sprays. Needles of Colorado blue spruce can be discolored (change from blue to green) by 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 budbreak. 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 not be applied to plants when there is danger of freezing. Also, dormant oils should not be applied to stressed plants, which are more susceptible to phytotoxicity. Lack of moisture, extreme 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 resistance. This may not be true. For example, a Christmas tree plantation of Scots pines was sprayed with dormant oils for over 10 years to control pine needle scale. Eventually the scales became 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 on. Treatments may not be needed in early spring, or the number of applications may be reduced. Reducing the number of insecticide applications pre-serves natural enemies of mites and scales, including parasitoids and predators, which usually supply sufficient control of these pests.