Horticultural oil fights overwintering pests

Many Las Vegas gardeners enjoy growing deciduous tree fruits and roses. The valley has a good climate for these rewarding plants.

As we head into spring, we need to clean up the debris around our plants to reduce many of the overwintering pests' hiding places. And to get rid of even more of those overwintering pests, make an application of horticultural oil. In my opinion, horticultural oil may be the most effective spray you will add this season to control pests.

But first, finish pruning these gratifying plants. Pruning improves a plant's form, removes dead, diseased wood and enhances fruit and flower production.

These productive plants also need a shot of an all-purpose fruit and rose fertilizer. Feed your fruit trees now and in April. Feed your roses monthly through October but cut the rates in half through the summer. Dennis Swartzell of Horticulture Consultants feeds his citrus trees on Valentine's Day, Memorial Day and Labor Day and they are loaded with fruit.

Let's now focus on horticultural oil, or dormant oil as we call it. Gardeners have used dormant oils as a major source of pest control for hundreds of years. Starting in the 1700s, petroleum oil, whale oil, kerosene mixed with soap and water, and other oils were the common uses for overwintering insects. They also are effective managing diseases such as powdery mildew.

Horticultural oil poses few risks to people and many of our beneficial insects. Because of this, oils often are integrated with other biological insect and disease control programs.

Horticultural oil toxicity is minimal because it quickly dissipates through evaporation, leaving little residue. And the oils are easy to apply, so you don't need a special sprayer to apply them.

Most horticultural oil products sold today are petroleum based. Modern refining techniques yield oils that are less harmful to plants and yet more harmful to insects. These 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 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 synthetic insecticides, so they are considered "safe."

Horticultural oils usually are sold as emulsified liquid concentrates or oil emulsions mixed with soap and water. The soap keeps the oil and water from separating to create very fine globules of oil surrounded by thin films of soapy water.

Between the 1930s and 1970s, oil products were highly viscous (thick) and recommended for dormant use only. That's why we call them "dormant oils," because they are applied to leafless plants. Sometimes this burned the plant's tissue. This was attributed to high concentrations of sulfur found in the crude oils.

You also may see what is called "summer oils" on nursery shelves. Today's horticultural oils (also called superior oils) are highly refined and much less viscous. This means you can apply them to foliage with little or no damage to plants in the summer.

Horticultural oils are effective at controlling many common pests. The most notable are aphids, leafhoppers, leaf rollers, mealy bugs, mites, psyllids, scales, tent caterpillars and fall webworm, particularly in overwintering stages.

Whenever using oils or pesticides of any kind on your plants, always read the entire product label three times: before you purchase it, once after getting home and each time before you apply it. Be very cautious applying oils when temperatures rise above 90 degrees. Oils increase the plant's transpiration rates (loss of water through the leaves), so water your plants before application during the summer.

Apply these special oils after leaves drop and you've cleaned up the area. But I find the best time to apply the oil is just before plants leaf or bloom out. That's when these pests are becoming active and when they are defenseless. If applied too early, make another application prior to buds opening.

SPRINGS PRESERVE'S NEW BEGINNINGS

On Saturdays and Sundays in January, there will be classes in gardening 101 (11 a.m.), fruit tree care (1 p.m.), caring for roses (2 p.m.) and tuning up your irrigation system (3 p.m.) as well as many other demonstrations at the Springs Preserve, 333 S. Valley View Blvd. For more information, including cost, go to www.springspreserve.org or call 822-7700.

Linn Mills writes a garden column each Sunday. You can reach him at linn.mills@ springspreserve.org or call him at 822-7754.