Even with the snow and fog, gardeners are already asking me how to control aphids on trees this spring, especially fruit trees. This is a timely question, because once temperatures rise to between 40 degrees and 70 degrees, it will be time to spray horticultural oil.
Although horticultural oil is a highly refined petroleum product, it is NOT motor oil or vegetable oil. It is made solely for use on plants. We use it to control soft-bodied insects, such as aphids and spider mites. The oil covers the insect eggs and overwintering insects, and smothers them. Often it is applied when plants are dormant, but by changing the rate, some products can be used even while plants are in leaf. Horticultural oil is an acceptable insect control for certified organic growers.
These products degrade quickly, with little or no environmental impact. Because of this short-term residual, they are less damaging to beneficial insects that help keep insect pest populations down.
Dormant oils are sometimes used in combination with fungicides. If doing this, be careful. Only certain oils can be combined with sulfur sprays, and only in very specific formulations. Otherwise, this combination can be extremely damaging to plants. Read the label and follow the label instructions.
Apply horticultural oils according to the rates listed on the labels to avoid plant damage. Problems can occur if the oil is sprayed too early in the fall or too late in the spring, or if buds are fully open or shoots are growing. Overdosing is never a good idea. Repeated applications within a short interval can be harmful.
Some plants are very sensitive and should never be sprayed with oils. Usually product labels identify these plants, but test-spraying on a very small portion of the plant is wise. Never spray blue spruce or other blue evergreens with oils, as the blue color will be lost, never to return on that portion of the plant.
A final word of caution about using summer oils: Spraying a tree in full leaf can seriously burn it when it is stressed from lack of water or summer's heat.
Never use a sprayer that has been used for a weedkiller to apply oil sprays or other insecticides. Spray when there is no wind. Spray the trunk and lower limbs completely, as most of the eggs are in the cracks and crevices of the bark and where limbs join the trunk.
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For more information, e-mail skellyj@unce.unr.edu or call me at 887-2252. You can "Ask a Master Gardener" by e-mailing mastergardeners@unce.unr.edu. Or call your local University of Nevada Cooperative Extension office. Check out many useful horticulture publications at www.unce.unr.edu.
JoAnne Skelly is the Carson City/Storey County Extension educator for University of Nevada Cooperative Extension.