tt-134 BY DON MEEHAN

HORTICULTURAL SPRAY OILS REDUCE PESTICIDE USE

For over one hundred years gardeners and commercial fruit growers have used horticultural spray oils to suppress insect and mite populations. For example, kerosene was widely used prior to the turn of the century to control aphid and scale insects. Today the kinds of oils available to the gardener and commercial grower have been improved to a high degree, yet not many people make full use of this special tool. What makes this particular tool special? One of the things we have learned about many of the pesticides used to control insects is that over time the insect population develops resistance to the material being used. This is not always the case, but can be particularly bad with specific pesticides. With horticultural oils we have never found any evidence of pest resistance to this material. This allows us to use it time after time without worry that it is losing its effectiveness. I would like to spend the rest of the time here addressing what horticultural spray oils are, how they work, and how you should use them. Like all materials used in the control of insect populations it pays to know all you can to avoid failure and to achieve the greatest level of success. Horticultural oils are highly refined crude oils. They are produced by separating all oils from crude oil based on their boiling point temperature, a distillation process. The lighter oils are those which have the lowest boiling points and are collected first since they come off the heated mixture first. Gasoline is an example of a very light oil. It boils at temperatures of 100 to 350 degrees F where as lubricating oils begin to boil at 700 degrees. Oils used as horticultural sprays begin boiling at a little over 500 degrees and seldom exceed 750 degrees. After the oil is separated, called fractioning, it is refined to remove toxic properties. Some oils are further refined to narrow the boiling point over a smaller range producing what is termed a "narrow cut" oil. Even though all spray oils go through similar treatments they can have considerable differences amongst them due to the chemical differences in the crude oil from which they were produced. Evaluation of spray oils generally falls into four categories, percent Unsulfonated Residues, distillation temp, viscosity, and degree of parafinicity. Unsulfonated Residue (U.R.) is a measure of the degree of saturation of the hydrocarbons in the oil. The higher this number the safer the oil is to plant material. Look for U.R. ratings on the product which are higher than 92%. Dormant sprays on apples and pears can have a lower U.R. of 80%. Distillation temperature or range determines effectiveness of the oil in killing the pest and reducing harm to the plant leaves. The newer narrow-cut oils are classified by a light to heavy grading standard. Each narrow range helps the user to better understand the oil's performance. Parafinicity is a measure of the amount of straight chain hydrocarbons present. Studies have shown that higher paraffin contents in spray oils produce better pest control performance. Viscosity is a measure of resistance to flow and relates to heaviness of the oil. Those oils that are more viscous tend to be more active pest killers. However, the trade off is that higher viscosities cause plant damage. HOW DO OILS KILL PESTS? There are three ways researchers believe oils act to kill pests. The first is that the oil smothers the insect by clogging its breathing apparatus. Since insect eggs require oxygen they too are susceptible to smothering. Second, there is some thought that the oil actually penetrates the cuticle layer of the insect and damages its nerve system. There is no data to support this however. Third, oil acts as a repellent by preventing egg laying in some insect species. HOW DO SPRAY OILS DAMAGE PLANT TISSUES? This is a complex issue from the start. Much of the damage done to plants in the past have been a result of using heavy oils with low U.R. values. It is important to remember that all plants must breath through their bark (lenticels) and their leaves (stomata). Oils can interfere with this air exchange. Damage may vary due to stage of plant growth or weather conditions at spray time. Plants which are actively growing are much more susceptible to damage as are trees under drought stress. Damage usually relates to three variables: plant stress, drying conditions, and concentration of the application. Spray oils can be an effective and safe tool if label instructions are followed to avoid damaging sensitive plants. They have been given the test of time and should be one of the first lines of defense against insect populations. If you want to learn more abut this special garden tool, call my office for a copy of WSU publication PNW328 Using Horticultural Spray Oils to Control Orchard Pests. It costs fifty cents and we will be glad to mail it to you. Another excellent companion publication is EB932 Insect and Mite Control in Home Orchard Tree Fruit and Nuts. It will give you details on when to use oils on what trees for maximum benefit.




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