Dormant Sprays 101

Dormant Sprays 101

Spring is the time to plan and begin your pest management program for fruiting and ornamental trees and shrubs. Many pests overwinter on woody plants in various life stages (egg, larva, fungal spores, etc.), and as they become active in the spring, they are vulnerable to treatment with a dormant spray. The spray could be oil alone, oil plus insecticide, or a fungicide. This treatment timing is important and effective because:

  • they are selective and environmentally friendly
  • they help prevent certain pest outbreaks when applied properly
  • individuals of the problem pest are all in the same life stage, such as an egg or larva, making them easier to target
  • pests are easily accessible
  • most natural enemies (insects and mites that prey upon pests) are not yet active, and so their populations are not harmed

What is Dormant Oil?

The most common dormant oil is petroleum-derived, and may also be referred to as horticultural oil, superior oil, supreme oil, paraffinic oil, and other names. This term describes a class of high quality oils formulated for agricultural use. Other oils, such as vegetable-derived oils, can also be used for a dormant oil spray. Any oil applied in early spring should be mixed with water to a concentration of 1.5 to 2% for best results. The same oil product may be applied in the summer, but should be mixed to a concentration of no greater than 1% to prevent plant injury.

What are the Characteristics of Petroleum-Derived Oil?

  1. Horticultural oils on the market today are considered “narrow range oils,” which means that the oil has been through two distillation stages, resulting in oil that has high insecticidal properties and low potential for plant injury.
  2. They are made from crude oil that is high in paraffin. Paraffinic oil comes from wells in the eastern U.S. and Texas. Studies have shown that paraffinic oils have the best insecticidal properties.
  3. The unsaturated hydrocarbons have been removed. These compounds were found in older oils and were responsible for plant injury. The term for oils low in unsaturated hydrocarbons is “unsulfonated residue.” The safer oils usually have a UR value of 92% to 97%.
  4. The label for oils will provide a variety of information:
    a. percent oil content (usually 98-99%): the oil may be listed as “mineral,”, “petroleum,” or “paraffinic.”
    b. minimum UR value: 92% or higher to reduce risk for plant injury.

How do Oils Kill Pests?

Oils kill pests primarily by smothering, and work best on soft-bodied insects. Insects require oxygen to live, and oil plugs the insect’s air-exchange system, causing slow suffocation. Oil works best on insect and mite eggs just before they hatch because their oxygen requirement increases.

Which Pests do Dormant Sprays Target?

mold   mold
coryneum blight                               peach twig borer
mold   mold
pear psylla eggs                                     aphid eggs

Common pests of woody plants in Utah that can be targeted with a dormant spray include many aphids, European red mite, rust mites, leafroller caterpillars, peach twig borer, scale insects, shothole borer, apple mildew, apple scab, and coryneum blight.

Can Other Chemicals be Added to Oil to Make it More Effective?

Oil alone is often inadequate to kill high populations of overwintered pests, so a supplemental insecticide may be needed. Pear psylla is a pest that is best managed by adding a synthetic pyrethroid, lime sulfur, or kaolin clay to the oil spray. And adding the insect growth regulator, pyriproxyfen (Esteem), or the organophosphate, chlorpyrifos (Lorsban), to the oil spray for scale insects also increases its efficacy. For rust mites, carbaryl (Sevin) plus oil, or sulfur alone at can be effective.

Note that some pesticides, including some formulations of sulfur, are incompatible with oil, so carefully read all product labels before mixing and applying to plants. Liquid formulations of pesticides, such as emulsifiable concentrate (EC), are most compatible with oil due to their ease of mixing.

What About Dormant Sprays for Diseases?

Fungal pathogens such as coryneum blight and peach leaf curl can be treated with a dormant spray of copper, chlorothalonil (Bravo), thiram, or ziram. Apple mildew can be treated with trifloxystrobin (Flint), lime sulfur, triflumizole (Procure), myclobutanil (Rally), fenarimol (Rubigan), and other fungicides.

When is the Ideal Time to Apply Dormant Sprays?

This will depend on the plant being sprayed as well as the target pest. For a few pests, the dormant spray should be applied while the buds on the plant are still tight (3 to 4 weeks before they typically start to swell). Pear psylla on pear trees, and coryneum blight and peach leaf curl on peach, nectarine, and apricot, are all active before bud break. Pear psylla adults spend the winter outside of pear orchards, returning to mate and lay eggs before buds begin to swell. A dormant spray will reduce the number of psylla adults, and eggs laid on buds.

The time to spray for most other pests that overwinter on trees is anytime during the period between bud swell and leaf emergence (i.e., when flower buds are exposed and/or when less than ½ inch of leaf is exposed). This period at bud break is called delayed-dormant, and may vary from tree to tree or location to location. At that time, insect and mite eggs begin to hatch, caterpillars emerge from hibernacula (overwintering structures), and other overwintering insects and diseases become active and more susceptible to pesticides.

How is the Spray Applied?

Because oil sprays kill by suffocating pests, thorough coverage of limbs and buds—until it begins to drip off—is critical for effective control. The tree limbs should look dark and slightly oily after the spray is applied. Thoroughly cover all limbs, but do not spray the lower trunk because beneficial predatory mites overwinter there and oil sprays can kill them.

For more information on dormant sprays, see Dormant Oil Demystified by M. Murray, Insect Control: Hort. Oils by Cranshaw and Baxendale, and Winter Pest Management in Backyard Fruit Trees by P.M. Geisel and D.C. Seaver.

-Diane Alston, Entomologist and Marion Murray, IPM Project Leader