This is the time of year to consider the possibility of using dormant oils to control insects and mites that survive the winter months in an overwintering stage, which includes eggs or mature females. Instead of waiting until spring to initiate control measures, making applications of dormant oil may be helpful in reducing pesticide and labor costs later in the season. Advantages of dormant oils include a wide range of activity against most species of mites and scales-even the eggs; minimal potential of insects' or mites' developing resistance; tendency to be less harmful to beneficial insects and predatory mites than other pest-control materials with long residual activity; and relatively safe to humans and other mammals. Disadvantages of using dormant oils include potential phytotoxicity during the growing season and 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). The unsulfonated residue is an assessment of the phytotoxic compounds remaining after distillation and refining. A high UR (> 92%) indicates a highly refined product with less potential for phytotoxicity. Dormant oils generally have a UR value < 92%.
Dormant oil applications are primarily directed at killing overwintering pests, such as mites and scales, before they are active in the spring and are capable of causing plant injury. Applications are performed during winter to minimize phytotoxicity to ornamental plants. A 2 to 4% rate is generally used in late fall to early spring. Dormant oils are contact materials that either suffocate, by blocking the breathing pores (spiracles), or directly penetrate and disrupt cell membranes of exposed insects and mites. Dormant oils have minimal residual activity once the material dries, so thorough coverage is essential.
Dormant oils are applied to all plant parts, which means that the overwintering stage of the insect or mite is located on the plant. However, not all insect and mite pests overwinter on plants. For example, dormant oil applications are not effective against twospotted spider mite, Tetranychus urticae, because this mite overwinters as a female in plant debris, mulch, or other non-plant protected places. In contrast, the spruce spider mite, Oligonychus ununguis, overwinters in the egg stage on plants, primarily evergreens such as arborvitae, juniper, hemlock, and pine, which means this mite is susceptible to dormant oil sprays.
Dormant oils are effective in killing the overwintering stages of scales, especially first- and second-instar nymphs or crawlers. For example, Euonymus scale, Unaspis euonymi, overwinters as a second-instar nymphs or mature female; both life stages are very susceptible to dormant oil sprays. However, scales that overwinter as eggs, such as oystershell scale (Lepidosaphes ulmi) and pine needle scale (Chionaspis pinifoliae) are more resistant to dormant oil applications. The primary reason is that eggs are generally stacked on top of each other, and the dormant oil may not penetrate and contact the bottom layer. As a result, additional insecticide applications after egg hatch are typically required.
A concern with the use of dormant oils is plant injury or phytotoxicity. Some plants, such as arborvitae, beech, redbud, and certain maples (Japanese, red, sugar, and amur), are highly sensitive to dormant oil sprays. The needles of Colorado blue spruce may be discolored-change from blue to green-as a result of a dormant oil application. Phytotoxicity is usually prevalent when higher rates (> 4%) are used and when applications are performed in early fall before dormancy or in late spring at bud-break. Problems with phytotoxicity are less likely to occur when applications are made in late October through February-when most plants are completely dormant. To avoid any potential problems due to phytotoxicity, it is important to make sure the spray solution is continually agitated.
Never apply dormant oils when there is the possibility of freezing. Dormant oils should be applied to deciduous plants when the ambient air temperature will stay above freezing for at least 24 hours. Evergreens are more susceptible to damage than deciduous plants, so making applications when temperatures remain above 40ºF over a 24-hour period is highly recommended. In addition, dormant oils should never be applied to plants that are stressed, as they are more susceptible to phytotoxicity. Lack of moisture, extreme temperatures, sudden changes in temperature after spraying, prolonged windy conditions, or disease or insect infestations may predispose plants to phytotoxicity.
There is a general “dogma” that using dormant oils is less likely to lead to insect and mite populations' developing resistance. However, this is not the case-remember, insects and mites don't read entomology books! 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 scale became more and more difficult to control. Guess what? It was discovered that the scale covers were thicker than normal, which made it harder for the dormant oil to penetrate and provide control.
Preventive dormant oil applications can save time and money later when dealing with insects and mites. Insecticide or miticide treatments may not be necessary, or the number of applications may be reduced, which preserves natural enemies of mites and scales, including predators and parasitoids that provide “free” control of these pests.