Keith Hansen

Winter time is a prime time to apply a dormant oil spray to deciduous fruit, nut and certain landscape trees and shrubs to control scales and other insect pests. Horticultural oils are highly refined petroleum products for controlling scale, mites and other overwintering insects and their eggs on plants, and work mainly by coating the pests with a suffocating film of fine oil.

Horticultural oils for controlling insect pests have been around for decades. Once upon a time, their only use was as a dormant spray on deciduous trees since formulations back then could injure plants when applied during the growing season. Great advances in formulations have been made and many horticultural oils are now formulated to be safely used, when manufacturer's directions are followed, during both the growing and dormant season.

Horticultural oils have several advantages over conventional pesticides. They have a wide range of activity against scale, mites, and other insects and their eggs. There is little or no resistance to oils by pests. A major advantage is that oils are usually less harmful to beneficial insects and predatory mites than other insecticides with longer residual activity. Oils are safe to handle and relatively harmless to humans, animals and birds, leave little or no residue on crops.

Some potential disadvantages of horticultural oils include injury to weakened or stressed plants when used during the growing season. Therefore, time applications to avoid high and low temperatures, drought conditions, and prolonged wind. Also, don't spray plants that are severely weakened by insect or disease. Read and follow labels carefully to avoid problems.

SCALES MOST SERIOUS

Scales are the most serious pests to control during the winter. Scales are tiny, sucking insects that attach themselves to tree limbs and branches with thin, smooth, tender bark. They suck sap from the plant and a heavy infestation of scale insects can weaken and kill branches or entire trees. Each scale insect encloses itself with a waxy, protective covering. This waxy armor makes scale difficult to control with most insecticides.

Most fruit trees including peaches, plums, apricots, apples and pears can potentially have scale infestations. Pecans can also benefit from an oil spray, although phylloxera insects are more of a problem than scale on pecan trees. If pecan leaves and leaf stalks had galls or large bumps on them last year - an indication of phylloxera infestation - then an oil horticultural oil application can help reduce the reoccurrence.

Camellias, holly and euonymus are three common landscape shrubs that can become infested with scales. Tea scales on camellia appear on the underside of leaves as small bumps covered with white. These evergreen plants can be sprayed in early spring when danger of cold weather is over, camellias have finished blooming, and scales become more active.

Late January or February, shortly before bud swell, bloom or leafing, is the best time to apply oil to dormant fruit trees. Scale insects grow weaker through the winter and are more vulnerable to the suffocating oil film if it is applied later in the dormant season. Do not apply dormant oil after trees have begun to have bud swell, bloom or leaf out.

Mix dormant oil according to label directions. Dormant oil works best if applied when the temperature is above 55 degrees although it can be applied when the temperature is between 40 and 70 degrees F. Do not apply when temperatures are predicted to go below 40 degrees.

Good spray coverage on the upper and lower sides of branches and leaves is critical for effective control. Use sufficient volume of solution to thoroughly wet limbs and bark.

A benefit of using dormant oil sprays is that they usually interfere less with natural insect control by beneficial parasites and predators than applications of conventional insecticides applied during the growing season.

Plants with a really severe scale infestation may need two oil sprays. If a second application is needed, wait at least 3 weeks between sprays. It can be difficult to tell if scales are dead since they don't move around, and dead scales don't fall of the bark or leaves. Take a knife blade or your fingernail and press on a scale. If a bit of "juice" comes out, it is alive. If the scales are flaky and dry, then they are dead. Scales with tiny holes have been parasitized by small wasps.

Keith Hansen
()