Oils can be effective in handling bugs

By ED PERRY
ejperry@ucdavis.edu

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The use of horticultural oil to control certain insect pests in deciduous fruit and landscape trees during winter fits well with home gardening. Along with reducing overwintering pests, horticultural oils are less toxic than many other insecticides, and much safer for beneficial insects such as bees and parasitic wasps. Oils can be combined with fungicides to control certain overwintering fungus diseases as well.

Horticultural oils are highly refined petroleum oils that are manufactured specifically to control pests on plants. They have several important advantages, including very low toxicity or almost no toxicity to humans or wildlife at the rates used to control pests. Properly used, they also have low toxicity to most plants. Horticultural oils break down rapidly through evaporation, so have little residual action that can affect beneficial insects. All of these qualities distinguish the horticultural oils from the more familiar motor oils and solvents, which can be toxic to wildlife and plants.

Technically, oil sprays are classified as physical contact insecticides. They kill insects and mites primarily by suffocation, although there may be additional toxic reactions in some insect species. As mentioned earlier, oils have almost no residual life, so they can kill only pests that are on the plant at the time of application. Insects that later crawl across the treated surface are not affected. Oils are most effective against eggs, immature insects and soft-bodied adults. Scales, mealybugs, aphids, leafhoppers, whiteflies, mites and eggs of all types of species are among the pests controlled. If you add a fixed copper fungicide to the mixture, you also can control such common and troublesome diseases as peach leaf curl and shothole disease. Be sure to read the product labels carefully for mixing instructions.

Horticultural oils available for plant protection once had a variety of names, such as dormant, summer, unclassified, medium, light-medium, etc., that signified various physical properties and degree of refinement. Today, virtually all horticultural oils available to home gardeners are identical and can be used for both dormant or growing season applications.

Most gardeners are familiar with the use of oils to control pests on fruit trees during the dormant season. However, fewer are familiar with the use of these oils during the summer season to control a broader spectrum of pests on all types of fruit trees and many ornamental species. Summer oil treatments can be used safely on many deciduous species and citrus, but check the label to be sure. Some plants may be damaged by oil sprays, including maple, walnut, spruce, juniper and some palms and ferns.

The time for dormant spray treatments is from December until the latter part of February. The term "delayed dormant" means February through as late as mid-March, provided flower buds have not begun opening. Once flower buds begin opening, you risk damaging them and killing the pollinating bees with the sprays.

As effective as they are, dormant sprays cannot control all of your fruit tree pest problems. For instance, overwintering codling moths, oriental fruit moths and navel orangeworms not affected by dormant treatments. Remember especially that codling moths, which give you wormy apples, cannot be controlled with dormant sprays. You still need to apply sprays during the growing season to keep that pest out of your apples, pears and walnuts.

Ed Perry, a farm adviser with the UC Cooperative Extension, can be contacted at 525-6800 or ejperry@ucdavis.edu.