Dormant Oil Spraying

Pinon Needle Scale

Tree Attacked By

Pinon Needle Scale

Dormant oil, diluted with water and applied as a spray, can be effective in controlling many plant pests. These horticultural oils are highly refined petroleum oils combined with an emulsifiying agent. The advantages of dormant oil include safety, effectiveness and limited effects on beneficial insects.

Dormant oil is an important tool to manage certain pest problems (e.g., scales, aphids, mites) on fruit trees, shade trees and woody ornamental plants. Several recently developed oils extend this usefulness to flowers, vegetables and other herbaceous plants. Dormant oil also can control some plant diseases, such as powdery mildew.

Dormant oil has different effects on pest insects. The most important is that it blocks the air holes (spiracles) through which insects breathe, causing them to die from asphyxiation. In some cases, the oil also may act as a poison, interacting with the fatty acids of the insect and interfering with normal metabolism. It may also disrupt how an insect feeds, a feature that is particularly important in the transmission of some plant viruses by aphids.

Dormant oil poses few risks to people or to most desirable species, including beneficial natural enemies of insect pests. This allows the oil   to integrate well with biological controls. Toxicity is minimal, at least compared to alternative pesticides, and it quickly dissipates through evaporation, leaving little residue. Oils also are easy to apply with existing spray equipment and can be mixed with many other pesticides to extend their performance.

Some plant pests controlled by horticultural oils.

Dormant Season Applications

* Aphids that curl leaves in spring

* Caterpillars that winter as eggs on the plant (leafrollers, tent caterpillars)

* Mites that winter on the plant (conifer-infesting species)

* Scale Insects (pine needle scale, striped pine scale, Kermes scale, cottony maple scale)

Summer/Foliar Applications

Insects and Mites

* Adelgids

* Aphids

* Eriophyid mites

* Leafhoppers

* Scale Insects

* Spider mites

* Whiteflies


* Powdery mildew

* Some aphid-transmitted viruses

Recommended Control

Mallory Landscape and Design recommends the application of dormant oil at a 1% by volume mixture to control non-beneficial insects and diseases. The first application is done before bud break, typically in late February or early March, depending on the weather. Further applications are done after plants and trees leaf out later in the spring and can continue throughout the growing season as required to control pests.

New Mexico Pesticide Applicator License #55493