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Dormant oil

Dormant oil

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Larry Sagers Horticultural Specialist Utah State University Extension Service Thanksgiving Point Office © All Rights Reserved

Most of us are accustomed to changing the oil in our cars and even oiling various pieces of mechanical equipment around the home. Ironically, ``oiling'' our trees and other woody plants is still a mystery for many gardeners. Various oils have been used for centuries to control certain insect and mite pests. Oils have recently regained popularity for managing scale, aphids and mites on fruit trees and other plants.

The popularity of oil has increased for several reasons.

Oils are an organic spray and are popular with gardeners who prefer to grow organically. Oils are relatively inexpensive when compared with other insecticides. Oils are exceptionally safe to people and non target animals, including bees. Oils do not produce objectionable odors and generally are not as damaging as many solvent base insecticides.

Obviously, a product with that many good points has some weaknesses.

Oils are strictly contact spray and must cover the pest to be effective. Spray oils have a tendency to cause plant injury or phytotoxicity in some situations. That means they will burn or otherwise damage the leaves of the trees. Oils may also stain some surfaces such as car finishes or dark colored house paints.

Oil sprays are particularly effective against the various scale insects that attack fruit trees and ornamentals. They offer partial control of eriophyid mites and other mites that commonly affect our trees. They are effective against the Cooley spruce gall adelgid and a variety of aphids. The reason that oils work well and insects have not developed resistance to them is that oils are not poisons.

Oils kill insects by suffocation. Insects breathe through tiny holes or pores in their exoskeleton. Spray oils plug these holes and prevent pests from breathing.

Oils are called dormant sprays, but this is misleading. Pests become active when temperatures rise above 50 degrees for several days in succession. Spraying too early means the insects are not active and are not as likely to be controlled by the spray. Trees must be starting to grow, with the buds opening slightly. Spray pome fruits when green tips show on the leaf clusters. Stone fruits are sprayed when flower color is visible.

Sprays should be completed before the trees start to blossom, as oils can affect pollination. If other insecticides are added, the mixture will be toxic to bees. Thorough application is extremely important. Sprays must cover the target pests that try to hide under the buds or rough bark. Avoid spraying the tree trunks and the soil surrounding the tree. Most predator mites overwinter on the tree trunks and in the soil. Spraying these areas increases problems with plant feeding mites.

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