Horticultural Oil Sprays - January 7, 2009
Jeff Schalau, Associate Agent, Agriculture & Natural Resources
University of Arizona Cooperative Extension, Yavapai County

Many Northern Arizona gardeners enjoy growing various deciduous tree fruits. The Verde Valley has a particularly good climate for peaches, plums, apples, pears, and other species. Once the harvest is over, fruit tree care continues. First, gather the mummies (rotten fruit) from the tree and soil surface to reduce overwintering pests (the mummies may be composted). An application of horticultural oil 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. Then, when the bloom is finished, you should apply a nitrogen fertilizer, preferably in three separate applications (May, July, and September).

Focusing back on the topic of horticultural oils, this pest control practice has been used for 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. Oils are also effective at managing some diseases such as powdery mildew.

Horticultural oils pose few risks to people and many beneficial natural enemies of insect pests. Because of this, oils are often integrated well with other biological insect and disease controls. Horticultural oil toxicity is minimal, at least compared to alternative pesticides, and oils quickly dissipate through evaporation, leaving little residue. Oils also are easy to apply with existing spray equipment and can be mixed with many other pesticides to extend their performance.

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.). Remember that horticultural oils are pesticides even though they are considered “safer”.

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. This sometimes 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 products are sometimes referred to as "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 fall webworm, particularly in overwintering stages.

Always read the entire product label before applying any product. Some products can be used in warmer weather. However, use them cautiously when temperatures are 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.

Getting back to your deciduous 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. Always apply pesticides in accordance with the label directions.

The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at cottonwoodmg@yahoo.com and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/.

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: December 29, 2008
Content Questions/Comments: jschalau@ag.arizona.edu
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