How a dormant oil spray will help eliminate tree pests

It’s officially spring, which means the flowers will be blooming and the pests will be working hard to damage your fruiting and ornamental trees and shrubs. Because several pests become active in spring, it’s important to make sure you use a treatment with a dormant oil spray to stop these pests at their most vulnerable stage. The following are just a few reasons why it’s important to get a dormant oil treatment:

  • 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
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Coryneum Blight

The most common dormant oil is petroleum-derrived, 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 spray. Any oil applied in early spring should be mixed with water to a concentration of 1.5 to 2 percent 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 percent to prevent plant injury.

How does a dormant oil spray 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.

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Peach Twig Borer

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

When is the best time to get a dormant oil spray?

There is not an exact time that applies to all pests. For some pests, the dormant oil spray should be applied while the buds on the plant are still tight (3 to 4 weeks before they typically start to swell). The time to spray for most other pests 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 1/2 inch of the leaf is exposed). 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.

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Aphid Eggs

The spray can be applied during the following range of bud stages:

Apples: swollen bud to 1/2” green

Pears: swollen bud to cluster bud

Peaches/Nectarines: swollen bud to pre-bloom

Apricot: before bloom

Keep in mind that dormant sprays are not required every year. For example if you had an aphid or peach twig borer problem last season on your peaches, then apply your delayed-dormant spray this spring. If you have had very little damage from those pests, then you could consider skipping the spray.

*Most of this post was used with permission from the Utah State University Extension.

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