If your fruit trees and berry bushes have had past problems with insect pests and mites, horticultural oils, applied this winter, could be a safe and effective approach to control.

Horticultural oils are refined petroleum products from which most plant-damaging impurities (sulfonated residues) have been removed. They are used during the winter (“the dormant season” when fruit trees and blueberry bushes lose their leaves and stop actively growing) to kill overwintering soft-bodied and slow-moving insects and mites.

Horticultural oils block the air-holes through which these pests breathe. They also penetrate the shells of insect and mite eggs and interfere with their metabolism and respiration.

Run-of-the-mill oils can harm plants because they contain toxic impurities and block carbon dioxide exchange which is important for photosynthesis. But horticultural oils are highly refined. When used as directed, horticultural oils rarely cause plant damage.

Horticultural oils have many benefits. They have little, if any, effect on birds, humans, pets and other mammals. They have limited impact on beneficial insects because beneficial insects usually aren’t active in winter. In addition, they do not have an objectionable odor during application and evaporate quickly, leaving no toxic residue.

On the downside, horticultural oils do not control all fruit tree and small fruit pests and diseases. They will not control apple maggot, codling moth, or pear slug. They will not control apple scab or brown rot. They are effective, however, in decreasing populations of overwintering aphids, mites, scale, mealy bugs and lace bugs.

If horticultural oils sound right for your orchard or blueberry patch, here are some tips for applying them:

• Because horticultural oils are most effective if pests are active and breathing, apply them just before leaf and flower buds show signs of swelling or breaking open (sometimes called a “delayed dormant stage”).

• Be sure to spray all parts of the plant including cracks and crevices, the top of the plant, the v-shaped crotch where limbs branch out from the trunk and any limb stubs left behind from pruning. Insects and mites can escape suffocation if oil coverage is poor.

• Apply dormant oils when temperatures are above 40 degrees. If applied during freezing weather, coverage will be uneven.

• Do not apply oils if plant tissues are wet or rain is likely. These conditions inhibit oil evaporation and can result in plant injury.

Avoid spray drift onto unintended plants when applying horticultural oils. Many herbaceous and some woody plants are sensitive to horticultural oils. (See sidebar.)

Spraying these plants with oils can result in blackened branches, yellowing foliage, leaf loss and even plant death.

Even though horticultural oils are very safe, they are pesticides. Therefore, always read the label before you use them and follow the directions for greatest success and safety.

Horticultural oil applied during the dormant season is a slick way to control aphids, mites and

other soft-bodied bugs. If you have had past problems with these pint-sized pests, consider using horticultural oils this winter.

Jeanette Stehr-Green is a WSU certified Clallam County Master Gardener.

Plants sensitive to horticultural oils

Avoid spraying horticultural oils on sensitive plants including the following:

Azaleas (selected varieties)

Beech*

Black walnut

Cedars

Cryptomeria

Douglas fir*

Hickories

Japanese holly*

Junipers*

Maples (particularly Japanese and red maple)

Photina*

Plume cedar

Redbud*

Smoke tree

Spruce*

* — Tendency toward sensitivity to horticultural oils