–by Dr. Raymond Cloyd
Dormant oils are applied during winter to kill insect and mite pests that survive the winter (overwinter) as eggs or mature females. Instead of waiting until spring to initiate management strategies, dormant oil applications can help reduce costs associated with pesticide inputs (in this case, insecticides and miticides) later in the growing season (spring through fall). The advantages of applying dormant oils include: 1) wide range of activity against the life stages of mite and scale pests—even the eggs; 2) less direct and indirect harmful effects to beneficial insects and predatory mites due to the timing of application; and 3) relatively low toxicity to humans and other mammals. The disadvantages of dormant oils include potential phytotoxicity (plant injury) and minimal residual activity or persistence.
Dormant oils are typically derived from paraffinic crude oil, and are the heaviest of the petroleum-based oil sprays with a low unsulfonated residue (Figure 1). Picture not available!
The unsulfonated residue is an assessment of the phytotoxic compounds remaining after distillation and refining. An unsulfonated residue >92% indicates a highly refined product with less potential for phytotoxicity. Dormant oils generally have a unsulfonated residue value <92%.
Dormant oil applications are primarily directed at killing overwintering life stages of certain mites and scales (Figure 2),
Fig 2. Overwintering life stage (adult female) of scale (Auth-Raymond Cloyd, KSU)
before they become active in the spring and feed on plants. Applications are made during winter to minimize phytotoxicity to ornamental plants. A 2% to 4% application rate is generally recommended in early winter to early spring. Dormant oils have contact activity and either suffocate by blocking the breathing pores (spiracles), or directly penetrate and disrupt cell membranes. However, dormant oils have minimal residual activity once residues dissipate, so thorough coverage of all plant parts is important.
Since dormant oils are applied to all plant parts, the overwintering life stage of insect or mite pests must be located on the plant. However, not all insect and mite pests overwinter on plants. For example, dormant oil applications are not effective against the twospotted spider mite (Tetranychus urticae) because the 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 conifers, such as; arborvitae, hemlock, juniper, and pine. Therefore, the spruce spider mite is susceptible to dormant oil spray applications.
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 because the eggs are generally stacked or piled on top of each other. Subsequently, dormant oils may not penetrate and contact the bottom layer. Consequently, supplemental insecticide applications are typically required after eggs hatch.
An issue when using dormant oils is the potential for phytotoxicity. Some plants, such as arborvitae, beech, redbud, and certain maples (Japanese, red, sugar, and amur), may be harmed by dormant oil sprays. Furthermore, the needles of Colorado blue spruce (Picea pungens) may be discolored or change from blue to green as a result of a dormant oil application. Phytotoxicity may be a problem when >4% application rates are used and/or when applications are conducted 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 late November through February, which is when most plants are completely dormant. To avoid phytotoxicity, always ensure the spray solution is continually agitated.
Never apply dormant oils when ambient air temperatures are ≤40ºF (≤4.4°C). Dormant oils should be applied to deciduous plants (trees and shrubs) when the ambient air temperature remains above 40°F (4.4°C) for at least 24 hours. Conifers, in general, are more susceptible to damage than deciduous plants, so it is best to apply dormant oils when temperatures remain above 40ºF (4.4°C) over a 24-hour period although there is no quantitative evidence suggesting that applications made at ≤40°F (≤4.4°C) will damage dormant fruit trees. In addition, dormant oils should never be applied to plants that are stressed since stressed plants are more susceptible to phytotoxicity. For example, lack of moisture, extreme temperatures, sudden drastic changes in the ambient air temperatures after spraying, prolonged windy conditions, and disease or insect infestations may predispose plants to phytotoxicity. However, there is no direct evidence indicating that dormant oils are harmful to stressed trees.
There is a general misconception that insect and mite pest populations are unable to develop resistance to dormant oils. However, this is not true. For instance, a Christmas tree plantation of Scots pine (Pinus sylvestris) trees was sprayed with dormant oils for more than 10 years in succession to “control” pine needle scale. Eventually, the scale population became more and more difficult to “control.” Why? Well, what was discovered was that the scale covers actually increased in thickness; making it difficult for the dormant oil to penetrate the outer covering and kill the eggs.
Dormant oil applications can alleviate dealing with insect and/or mite pest populations during the growing season. Therefore, inputs from insecticide and/or miticide applications can be reduced, thus preserving the natural enemies of mites and scales, including; predators and parasitoids that may naturally regulate populations of these pests.