   Fruit Trees and Horticultural Oils - October 18, 2000 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County Harvest your apples and enjoy them, but don't simply abandon your orchard until next spring. First, you will need to pick the mummies (rotten fruit) from the tree and off the ground to reduce overwintering pests (these may be composted using a hot method). Next, you may decide to apply horticultural oil that will further reduce overwintering pest populations. In late winter/early spring, most fruit trees can be pruned to improve form, remove dead wood, and enhance fruit production. In spring, nitrogen fertilizer should be applied to improve fruit quantity and quality and you are off to the races for another crop. For this week, lets assume you've dealt with your mummies and focus in on the use of horticultural oils to reduce insect populations. Use of oils to control pests goes back over 1,900 years. Pliny the Elder wrote that mineral oil controlled certain plant pests. Starting in the 1700's, petroleum oil, whale oil, kerosene mixed with soap and water, and other oils were in common use for insect population control. Most horticultural oil products sold today are petroleum based. Modern refining techniques yield oils that are less harmful to plants and more harmful to insects. Today's horticultural oils are a complex mixture of hydrocarbons containing traces of nitrogen- and sulfur-linked compounds. Newer products have been developed from vegetable products such as safflower and corn. In general, horticultural oils kill insects either by blocking their breathing apparatus, interfering with their metabolic processes, or preventing respiration through their shells. These modes of action are mostly physical and do not promote resistance like other synthetic insecticides that disrupt the insect (and other non-target species) nervous systems (i.e. malathion, diazinon, etc.) Horticultural oils are usually sold as emulsified liquid concentrates or oil emulsions. These are mixtures of oil, soap, and water. Here, the soap helps keep the oil and water from separating. This creates a mixture of very fine globules of oil surrounded by thin films of soapy water. Between the 1930's and 1970's, oil products were highly viscous (thick) and recommended for dormant use only. These products were called "dormant oils". They controlled over-wintering pests on dormant, leafless, deciduous trees. However, when applied, they could also clog plant stomata and lenticels along with pores of the insects. This caused undue stress on the plant and burning of tissue. This was compounded by high concentrations of sulfur containing compounds that carried over from the crude oil. Today's horticultural oils (also called superior oils) are highly refined and much less viscous, they can be applied to leaves and stems with little or no damage to the plant. These are often called "summer oils". Horticultural oils are effective at controlling many common ornamental and fruit tree pests. These include: aphids, leafhoppers, leafrollers, mealybugs, mites, psyllids, scales, tent caterpillars, and webworms, particularly in overwintering stages. Always read the entire product label before applying any product. Two percent solutions of horticultural oils can also be used during the summer months. Caution should be exercised when the temperature is above 90 degrees F. The oil tends to increase the plant's transpiration rate (loss of water through the leaves). So, it is critical to avoid using horticultural oils in summer if there is not sufficient soil moisture. Blue spruces and other conifers with a waxy or powdery leaf surface often loose their color because the oil removes this coating and it can take two to three years before returning to normal. Horticultural oils work well when used to control pinyon needle scale during the growing season. Horticultural oils can adversely affect thin-barked species, such as maples, beeches, and birches. If you are unsure, then apply the appropriate solution to a small portion of the plant observing it for damage. If no damage occurs within two weeks, then the plant will likely tolerate that product concentration under those weather conditions. Getting back to your fruit trees, apply horticultural oil after all leaves have dropped in fall and mummies have been removed. Some gardeners repeat the application in late winter. This will assist in controlling overwintering fruit tree pests. The University of Arizona Cooperative Extension has publications and information on pest control and fruit tree care. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 or E-mail us at mgardener@kachina.net and be sure to include your address and phone number. The Yavapai County Cooperative Extension web site is http://ag.arizona.edu/yavapai/. |