Winter time is prime time to apply, if needed, a dormant oil spray to deciduous fruit, nut and certain landscape trees and shrubs to control scale and other insect pests. Horticultural oils are highly refined petroleum products for controlling scale, mites and other overwintering insects and their eggs on plants. Horticultural oils work mainly by coating pests with a suffocating film of fine oil. Their toxic action is more physical than chemical and is short-lived.
Horticultural oils for controlling insect pests have been around for many decades. Initially, their only use was as a dormant spray on deciduous trees since early formulations could injure plants when applied during the growing season. Great advances in formulations have been made and horticultural oils are now 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, and some formulations can be used by organic farmers in the TDA Organic Certification Program, if they are OMRI listed.
Some potential disadvantages of horticultural oils include injury to weakened or stressed plants when used during the growing season. Therefore, time applications during the growing season to avoid high temperatures, drought conditions, and prolonged wind, and don’t spray plants severely weakened by insect or disease. Always read and follow labels carefully to avoid problems.
Scales are the most serious pest to control in the winter time. Scales are tiny sucking insects that attach themselves to tree limbs and branches with thin, smooth, tender bark. There are also scales that will feed on leaves. They suck sap from the plant and a heavy infestation of scale insects can weaken and potentially kill branches or entire trees. Each scale insect covers itself with a waxy, protective material. This waxy armor makes scale difficult to control with most insecticides.
Scale is often overlooked because they often blend in to branch and bark color. Look for the presence of small bumps that can be flicked off with a fingernail.
Most fruit trees including peaches, plums, apricots, apples and pears can have scale infestations. Pecans can also benefit from an oil spray, although phylloxera 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 a horticultural oil application can help reduce the reoccurrence.
Euonymus and camellia are two shrubs frequently affected by scale insects on their leaves. The Camellia Tea Scale (so named because the tea we drink comes from a camellia species) can be particularly damaging and difficult to kill.
A newcomer scale to our area is the Crapemyrtle Bark Scale (CMBS), which is rapidly spreading across the south. The tell-tale sign of CMBS are black trunks, a result of sooty mold growing on the scales’ sugary exudate. A dormant oil spray is one tool to help control this pest, though it is not a complete control solution.
As I mentioned earlier, dormant oil sprays are very “soft” on beneficial insects. But, if insect predators (in the adult, larvae or pupal stage) are also present, like the lady beetles that have been found eating Crapemyrtle Bark Scale, then they too would be affected. So, check your plants carefully for not only the presence of pest insects, but also for beneficials.
Late January or February, shortly before bloom or budbreak, is the best time to apply oil. Scale insects grow weaker through the winter and are more vulnerable to the suffocating oil film if it is applied late in the dormant season. Do not apply dormant oil after trees have begun to bloom or leaf out.
Mix 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 use when the temperature is predicted to fall below 40. As with any pesticide, always read, understand and follow all product label directions.
Complete spray coverage on the upper and lower sides of branches is critical for effective control. Use sufficient volume of solution to thoroughly wet limbs and on and under bark.
Trees 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 finger nail 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.