Dormant trees may not be so easy on the eyes for us, but overwintering insects still find their look appealing. Throughout the growing season, scale insects, mites, and aphids will lay eggs on trees that persist through the winter months until new larvae are born. These larvae can damage trees, as well as certain shrubs.

Thankfully, you can help protect your trees and shrubs with dormant oils (also known as horticultural oils), a readily available and relatively inexpensive solution. An application of dormant oil will help control overwintering insect populations by physically coating the egg surface and/or the larvae’s spiracles (breathing holes), effectively suffocating the insects. Best of all, oils are considered relatively safe for the applicator and the environment. The oils are less harmful to beneficial insects, such as ladybugs, and are relatively nontoxic to birds and mammals.

Applying dormant oil will help to keep your trees and shrubs healthy and give you a head start on insect management in the spring. They provide a good foundation for a successful plant health care season.

Here are four tips to maximize the effectiveness of an application of dormant oil:

  1. Timing is everything. Dormant oils should not be applied until a tree has gone completely dormant, but also before new growth occurs. Applications in late fall or early spring are ideal when temperatures are above freezing (over 40 degrees F is ideal), but also below 70 degrees F. Applying the oil when the weather is too cold may expose your tree to winter damage. Concentrations of oil must be lower than full strength when used during the growing season, or when temperatures rise

  1. Achieve full coverage. The dormant oil mode of action is the physical, not chemical, disruption of insect breathing. Therefore, ensuring a thorough spray coverage so that all harmful insects are smothered with oil residue is critical. Special equipment, knowledge of the pest, and/or consideration of plant structure may be necessary.
  2. Coordinate with other applications. If you also apply a sulfur-containing pesticide, be sure to space out your applications. A combination of dormant oil and a sulfur-containing pesticide can create a combination that is toxic to plants. To play it safe, wait 30 days after a sulfur pesticide application before using dormant oil. Applicators should be informed on what products can and cannot be tank-mixed with oils.

  3. Hire and Consult with a Certified Arborist. Timing, proper spray coverage, and coordinating with other applications can be daunting. Hiring an experienced, certified arborist will ensure your trees are in good hands. Arborists specialize in tree care and have the knowledge, training, and proper equipment to provide proper care.

To receive more tips and insights, contact your local Davey arborist today.

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