Applying Dormant Oils Can Help Protect Your Trees From Parasitic Insects

Applying Dormant Oils Can Help Protect Your Trees From Parasitic Insects

While dormant trees may not be so easy on the eyes for us, overwintering insects still find their look appealing. Late in the summer, scale insects, mites and aphids will lay eggs on trees that stay on through the winter months until new larvae are born. These larvae can damage fruit trees, such as apple and pear trees, as well as certain shrubs and woody ornamentals.

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 coating the insects’ spiracles, effectively smothering future larvae. Best of all, the oils are less toxic to beneficial insects, such as lady bugs, as well as 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.

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 winter or early spring are ideal, when temperatures are above freezing (over 40 degrees is ideal), but also below 70 degrees. Applying the oil too early may expose your tree to winter damage. Applying too late will damage new growth.

  1. 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 compound poisonous to plants. To play it safe, wait 30 days after a sulfur pesticide application before using dormant oil. 

  1. Check the label. When choosing a dormant oil, look for a low-viscosity oil, which will spread more easily. Make sure the product you choose is specifically marked as a dormant oil, as other oil types can damage trees and shrubs. Also, be aware of the types of trees and shrubs your dormant oil is formulated to protect. A dormant oil for your apple tree might be harmful for a woody ornamental elsewhere on your property.

  1. Apply with caution. Follow all instructions on the label when applying a dormant oil and be aware of your tree or shrub’s surroundings. If you aren’t sure how or when to apply, contact a local, licensed professional. Nearby annual flowers in particular can be damaged.

To receive more tips and insights on growing and maintaining healthy trees and shrubs directly in your inbox, sign up for Davey Tree’s e-newsletter.

  • The Tree Doctor November 8, 2016 >Hi there, Darren. Good question. Any formulation of horticultural oil will strip the blue color from your blue spruce. It won't hurt the tree, but it will turn it green. If you have a green spruce, any dormant oil will work. At Davey, we use LESCO Horticultural Oil. Hope this helps, Darren. Wishing you and your trees well!
  • Darren Hamer November 1, 2016 >Can you please explain the name of the dormant oil that is used so I can perform reading about its application effectiveness for blue spruce via google searches. Thank you.
  • Gloria Sephers October 23, 2015 >I have an oak tree - about 5 years old - that I think is planted too close to my house. What would it cost be for your assessment of this situation and relocation of the tree, if needed?
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