Insects and mites normally survive the cool winter months as some type of overwintering stage, which may be an egg or mature female, and then emerge in the spring. Instead of waiting until spring to initiate control measures, it is possible to make applications of a dormant oil. The advantages of dormant oil include (1) wide range of activity against most species of mites and scales, including effectiveness on eggs; (2) minimal probability of insects or mites developing resistance; (3) tendency to be less harmful to beneficial insects and predatory mites compared to other insecticides with long residual activity; and (4) relatively harmless to humans, other mammals, and birds. The disadvantages of dormant oils are (1) phytotoxicity during the growing season and (2) less persistence and minimal residual activity.

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 the level of phytotoxic compounds remaining after distillation and refining. A high UR (>92%) indicates a highly refined product with less probability of phytotoxicity. Dormant oils have a UR value below 92%.

Dormant oil sprays are directed primarily at killing the overwintering stages of pests such as mites and scales before they become active in the spring and can injure plants. Applications are made during the winter months to minimize phytotoxicity to plants. Generally a 2% to 4% rate is used in the late fall to early spring for killing insect and mite pests. Dormant oils suffocate insects or mites by blocking their breathing pores (spiracles). These 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 overwintering stage is located on the plant. However, not all insect and mite pests overwinter on plants. For example, dormant oil will not work on the 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, does overwinter as an egg on plants, especially 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 scale, primarily the 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, those that overwinter as eggs such as oystershell scale (Lepidosaphes ulmi) and pine needle scale (Chionaspis pinifoliae) may be difficult to control. The eggs are generally stacked on top of each other, and the dormant oil may not contact the bottom layer. This means that additional sprays after egg hatch are generally required.

A concern with the use of dormant oils is phytotoxicity. Some plants such as arborvitae, beech, and certain maple species are very sensitive to dormant oil sprays. The needles of Colorado blue spruce can be discolored (change from blue to green temporarily) 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 late spring (at budbreak). Fewer problems occur when applications are made in later October through February when plants are completely dormant. A way to minimize potential phytotoxicity is to make sure that the spray solution is continually agitated. Dormant oils should not be applied to plants when there is a danger of freezing. In addition, dor-mant oils should not be applied to stressed plants. Stressed plants are 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 will predispose plants to phytotoxicity.

It is generally thought (or assumed) that using dor-mant oils is less likely to result in resistance. However, this may not necessarily 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 diffi-cult to control. It was discovered that the scale covers were thicker than normal thus making it harder for the dormant oil to penetrate the covering.

Preventive dormant oil sprays can save time later on as treatments may not be needed in early spring, or the number of applications may be reduced. In addition, this minimizes spraying with pest-control materials that are harmful to natural enemies of mites and scales.