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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. First, oil is an organic spray and is popular with gardeners who prefer to grow their crops organically. Second, oil is relatively inexpensive when compared with other insecticides. Third, oils are exceptionally safe to applicators, other individuals and non-target animals, including bees. They do not produce objectionable odors and generally are not as damaging to sprayers 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 reasons 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.

Oils can be purchased in several weights or degrees of viscosity. The safest are oils that are lighter in weight, more refined and contain less sulfur. Oils sold to the retail garden trade are very pure and cause few problems with plant injury. Exceptions to the plant injury rule include blue spruce and several other plants. Ferns, Japanese maples and many herbaceous plants are oil sensitive. Certain maples and walnuts are sensitive to the higher weights of oils. Oils should not be applied if temperatures are very high or extremely low. Never add sulfur, including lime sulfur, near oil applications.

Despite the drawbacks, oils are effective on many early-season pests. Predators of these pests often don't become active until after pest populations have developed. Using oil sprays makes it possible to reduce early-season outbreaks and reduce curled leaves and other damage that occurs from these infestations. Pests are generally very difficult to control after infestations occur because the leaves are curled and the insects are protected inside those curled leaves.

Don't misunderstand what a dormant spray is. Dormant sprays for our area are horticultural spray oils (Volck, Superior, Supreme or other brands) that you mix with an insecticide. Insecticides increase the effectiveness on early-season pests, but the oils without the insecticides can be used by organic gardeners. Dormant disease control is a lime-sulfur spray and, as mentioned, should not be used in conjunction with oil sprays. Lime sulfur is formulated to control diseases, not insects. It is generally not needed in our area, as we don't get peach leaf curl and other diseases it controls.

RED BUTTE GARDENS - HORTICULTURAL CLASSES, 290 Wakara Way, instructor Larry Sagers, USU Extension horticulturist. Preregistration is required, call 581-5322.

- Rose care: Wednesday, April 1, 5-7 p.m. Red Butte Gardens.

- Pruning trees: Wednesday, April 15, 5-7 p.m. Red Butte Amphitheater.

- Pruning shrubs, vines and ground covers: Wednesday, April 22, 5-7 p.m. Red Butte Amphitheater. FARM AND GARDEN SHOW - Wheeler Historic Farm and KSOP Radio will present the second annual Farm and Garden Show on Saturday, April 4, from 10 a.m. until 5 p.m. at Wheeler Farm. During show hours, admission to Wheeler Farm is free.

Exhibitors will have their products on display in the Activity Barn as well as outside on the farm grounds.

The activities of Wheeler Farm will continue, with skill demonstrations, children's games and tours of the farmhouse. Horse-drawn wagon rides will be offered at $1 per person from 9 a.m. until 4 p.m. A bungee-cord jump, "Leap Into the Historic Past" will be sponsored by Total Recoil. A fee will be charged for this event, with $5 from every jump going to Wheeler Farm.

The farm is located at 6351 S. 900 East. For further information, call 264-2241.