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The use of dormant oils is a pest management tactic designed to deal with insect and mite pests that survive the winter in an overwintering life-stage, which may include eggs or mature females. Instead of waiting until spring to initiate “control” measures, applying dormant oils may be helpful in reducing costs associated with pesticide (in this case, insecticides and miticides) use later in the season. The advantages of applying dormant oils include a wide range of activity against most species of mites and scales—even the eggs; minimal potential for resistance developing in insect and/or mite pest populations; less directly and indirectly harmful to beneficial insects and predatory mites compared to pesticides with long-residual activity; and relatively non-toxic to humans and other mammals. The disadvantages of dormant oils include potential to harm or kill plants during the growing season (phytotoxicity) and minimal residual activity or less persistence.
Dormant oils, which are typically derived from paraffinic crude oil, are the heaviest of the petroleum-based 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 (greater than 92 percent) indicates a highly refined product with less potential for phytotoxicity. Dormant oils generally have a UR value less than 92 percent.
Dormant oil applications are primarily directed at killing overwintering pests including mites and scales, before they become active in the spring, and are capable of causing plant injury. Applications are made during winter to minimize phytotoxicity to ornamental plants. A 2- to 4-percent rate is generally recommended in late fall to early spring. Dormant oils have contact activity and either suffocates, by blocking the breathing pores (spiracles), or directly penetrates and disrupts cell membranes of exposed insect and mite pests. However, dormant oils have minimal residual activity once residues dissipate, so thorough coverage is essential.
Dormant oils are applied to all plant parts, which means that the overwintering stage of the insect or mite pest must be located on the plant. But not all insect and mite pests overwinter on plants. For example, dormant oil applications are not effective against the two-spotted 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 as an egg on plants, primarily evergreens such as arborvitae, hemlock, juniper, and pine, which means that this mite species is susceptible to dormant oil sprays.
Dormant oils are effective in killing the overwintering stages of scales, especially first and second instars or nymphs (=crawlers). For example, euonymus scale (Unaspis euonymi) overwinters as second instar nymphs or mature females; both life stages are susceptible to dormant oil applications. However, certain scales that overwinter as eggs, such as oystershell scale (Lepidosaphes ulmi) and pine needle scale (Chionaspis pinifoliae), are more tolerant of dormant oil applications. The reason is that the eggs are generally stacked or piled on top of each other, and the dormant oil may not penetrate and contact the bottom layer. As a result, additional insecticide applications are typically required after egg hatch.
An issue when using dormant oils is the potential for plant injury or phytotoxicity. Dormant oils sprays may harm some plants such as arborvitae, beech, redbud, and certain maples (Japanese, red, sugar, and amur). Furthermore, the needles of Colorado blue spruce may be discolored or change from blue to green as a result of a dormant oil application. Phytotoxicity is usually a problem when higher rates (for example, greater than 4 percent) are used and/or when applications are performed in early fall before dormancy or in late spring at bud-break. Problems associated with phytotoxicity are less likely to occur when applications are made in early November through February when most plants are completely dormant. In order to avoid phytotoxicity it is important to make sure the spray solution is continually agitated. Also, never apply dormant oils when there is a possibility of freezing (temperatures at or below 32ºF). Dormant oils should be applied to deciduous plants when the ambient air temperature will stay above freezing for at least 24 hours. Evergreens, in general, are more susceptible to damage than deciduous plants, so it is best to apply dormant oils when temperatures remain above 40ºF over a 24-hour period. Furthermore, dormant oils should never be applied to plants that are stressed because stressed plants are more susceptible to phytotoxicity. Lack of moisture, extreme temperatures, sudden drastic changes in the ambient air temperatures after spraying, prolonged windy conditions, or disease or insect infestations may predispose plants to phytotoxicity.
There is a prevailing notion that insect and mite pest populations cannot develop resistance to dormant oils. But this is not true. For example, a Christmas tree plantation of Scots pines was sprayed with dormant oils for more than 10 years in a row to “control” pine needle scale. Eventually, the scale population became more and more difficult to “control.” Why? It was determined that the scale covers actually increased in thickness, which made it hard for the dormant oil to penetrate the outer covering and kill the eggs.
Preventative dormant oil applications may avoid dealing with abundant insect and/or mite pest populations later on during the season. Input from insecticide and/or miticide applications may be reduced, which preserves the natural enemies of mites and scales, including predators and parasitoids that naturally regulate populations of these pests.
John Hobbs, Extension Agent
McDonald and Newton Counties
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