Dormant oils help control unwanted insects such as scales. In Harmony Sustainable Landscapes.

Dormant oils help control unwanted insects such as scales. Photo: James Solomon, USDA Forest Service,

Some unwanted insects and mites survive the winter on the bark of trees and shrubs. In Harmony applies dormant oils as needed in winter to manage these pests.

We use dormant oils to control soft-bodied pests such as scales, mites and aphids. Oils are commonly used on fruit trees and some evergreens.

“Dormant” refers to the time of year when the oils are used, not the oils themselves. During winter dormancy neither the plants nor insects are actively growing.

Dormant oils are usually refined petroleum oil, cottonseed oil, canola oil or a similar product. We use an OMRI certified mineral oil for our dormant sprays. OMRI certification means it can be used by organic farmers and producers. The oil we use is both an insecticide and mild fungicide.

The oils work by smothering the pest. They may block the pests’ breathing pores, penetrate and destroy their cells or desiccate them (dry them out). The oils need to contact the insect or mite directly to work. It may look like we are spraying a tree or shrub heavily because we need to reach all of the plant’s nooks and crannies.

Dormant oils have a low toxicity to humans, birds, beneficial insects and animals. They don’t leave a harmful residue. They lose their ability to control pests once dried, however.

The oils will not work on all overwintering insects. It depends on where they spend the winter. For example, the spruce spider mite overwinters as an egg on evergreens such as arborvitae and juniper. So, dormant oil sprays will be effective against this pest.

On the other hand, the twospotted spider mite overwinters in plant debris or mulch, not on a tree branch or trunk. It would be difficult to find and treat this pest.

Dormant oils are very effective at killing most overwintering scales. However, scales that overwinter as eggs are tougher to control. This is because eggs are usually stacked on top of the each other, and the dormant oil may not contact the bottom layer.

Dormant oils are unlikely to harm beneficial insects because they are applied at a time when beneficial insects are not around. This means that natural enemies of mites and scales will survive to help control these pests.

Some plants can be harmed by oil sprays. In Harmony takes care not to treat plants that may be harmed by oil sprays.

Contact us if you would like to know more about our natural tree and shrub care services.


Dormant oils: how to match spray with pest, The Seattle Times.

Homemade Dormant Oil Spray for Fruit Trees, SFGATE.

Just what is dormant oil?, University of Illinois Extension.

Why Use Dormant Oils?, University of Illinois Extension.









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