Marin Master Gardeners
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Horticultural oils battle bugs nontoxically

June 11, 2011
Martha Proctor

IS MUCH OF THE FRUIT on your trees being consumed or damaged by pests? With the number of insect species in California estimated to be between 30,000 and 35,000, it is no wonder that it is an ongoing battle to keep the plants in our gardens protected from pests.

A plant pest is defined as a living creature that either feeds on or harms a plant. The damage may range from occasional nibbling of leaves to killing the plant. The damage done by larger pests — slugs, deer, rabbits and other leaf chewers — is easy to see and in some cases, putting up a fence will protect vulnerable plants.

Insects in their larval stage that feed on plants may metamorphose into a completely different adult form, such as caterpillars to butterflies, grubs to beetles.

A pest's feeding habit will determine how it damages a plant and what part of the plant is affected. Pests with chewing mouthparts eat portions of the plant. They may defoliate the plant by eating all the leaves. They may only eat portions of leaves, resulting in skeletonized, notched, shot-holed or shredded foliage. Other chewing pests feed inside leaves (leaf miners), or bore into stems and roots (borers).

Pests with sucking mouthparts usually cause the leaves of the plant to discolor or twist and curl. The leaves may discolor from tiny yellow speckles (spider mites), larger darkened spots (plant bugs), or coatings of black sooty mold growing on honeydew deposits (aphids, whiteflies).
Many plants react to the saliva and damage of sucking pests by causing the foliage to curl or the young stems to twist. For example, aphids suck sap from the plant and can cause new growth to be stunted and distorted; thrips "rasp" into leaves to obtain the plants juices, leaving the leaf distorted and with noticeable scars. Coddling moths lay their eggs close to a fruit tree so that after the young larvae hatch, they can bore into the fruit to feed.

Oil-based pesticides, horticultural oils, are an effective and ecologically friendly way to handle many garden insect pests and some diseases. Written records on the use of oils as pesticides date back from as early as the first century A.D., when Pliny the Elder wrote that mineral oil controlled certain plant pests. By 1763, petroleum oil and turpentine were in common use as insecticides.

More recently, oils became popular as a way to control pest problems on fruit trees, as oils are toxicologically much safer than most other pesticides. Oils also have a relatively short residual life and less impact on beneficial insects than other synthetic products. Horticultural
oils are only effective against soft-bodied, immobile or slow-moving pests that are thoroughly coated by the spray.

The majority of pest control oils in use today are a type of mineral oil, a refined petroleum product. There are also a few vegetable oils such as cottonseed, sesame and soybean oils, that are also effective pesticides. The oil is combined with an emulsifying agent so that it can be mixed with water and used as a spray.

Horticultural oils available in garden centers today are highly refined oils that are sprayed at a 2 percent solution on plants in full leaf if or when environmental conditions are right. Use of previous, heavier, more impure oils often resulted in clogged breathing pores.  Newer, lighter, more plant-friendly products, although still called superior, summer, narrow-range or supreme oil, must contain at least 92 percent of unsulfonated aromatics and no more than 8 percent sulfonated aromatics/impurities.  Dormant oils should only be used in the dormant season, as that is how/when they are formulated to do best.

Horticultural oil sprays kill all stages of insects by blocking the spiracles through which insects breathe. These oils also disrupt the metabolism of insect eggs and the ability of some insects to feed, causing them to starve to death. Insects killed or repelled by horticultural oils include aphids, leafminers, leafhoppers, mites, sawflies, scale, psyllids, whiteflies, tent caterpillars, mealybugs and even powdery mildew.

To reduce the chance of damaging the plant when using horticultural oil, spray in the early morning or on cloudy days when the relative humidity is low to moderate and the temperature is between 40 degrees and 70 degrees. Irrigate thoroughly before spraying to ensure that the plant is
not water stressed. The cooler and shadier the conditions the day the oil is
applied, the better.

Avoid using the oils on plants weakened by disease, drying winds or high nitrogen applications. As with any insecticide, test a small section of the plant before treating the entire specimen. Pick ripe fruit or vegetables, prune plant if appropriate and remove weeds that provide pest cover.

Some pesticide labels specify mixing percentages but don't include practical information for getting those percentages in volumes that people are used to dealing with. If you're trying to calculate percentages for mixing, remember that one cup equals 1/16th of a gallon, or about 6 percent. So, if your label calls for a 2 percent solution by volume, you could mix cup oil into a 1gallon container and then fill the container with water.

Horticultural oil is a labeled pesticide and should always be used according to the label's instruction. Avoid treating plants during the fall as fall treatments have sometimes caused increased susceptibility to winter injury. Don't spray if plant tissues are wet or rain is likely. Horticultural oils can kill annual flowers, which are growing under or near the treated plant. Avoid exposing black walnut, Douglas fir, junipers, cedars, redbud, cryptomeria, hickory, Japanese maples, spruce and smoke trees to horticultural oils as they are sensitive to the spray.

A few ounces of prevention applied at the right time under the proper conditions can protect your fruit trees and many other plants from some pest damage. It is a very frustrating experience to put all the time and money into your garden only to have the fruits of it ruined by hungry pests.

Oil spray repellants

Horticultural oil sprays kill all stages of insects by blocking the spiracles through which insects breathe. The oils also disrupt the metabolism of insect eggs and the ability of some insects to feed.

Insects killed or repelled by oils include: aphids, leafminers, leafhoppers, mites, sawflies, scale, psyllids, whiteflies, tent caterpillars, mealybugs and even powdery mildew.

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