This time of year you may be hearing about dormant oil spray from online sources or your gardening friends.  Dormant oils are used to kill mites and insects, such as scales and aphids that spent the winter on the plant. They work by covering them in oil that plugs their breathing apparatus and suffocating them. For the home owner, dormant oils should only be used when you have experienced these insect problems in past growing seasons and you want to get ahead of the issue for the coming season by reducing the population that is overwintering in your garden.  

Dormant oils are usually refined petroleum oils, commonly known as mineral oil, combined with an emulsifying agent.  Organic gardeners can use a dormant oil derived from refined vegetable oil. These oils are mixed with water according to label instructions and applied as a spray.  The emulsifier in the blend is used to keep the oil suspended in the water.  Dormant oils, as implied by the name, are intended to be applied as a dormant-season application (before bud swelling and bud break). 

Historically a dormant oil could only be applied to woody plants during their dormant period however improvements in refining of the mineral oils has allowed for horticultural oils that can be used at other times of the year as well as on plants with foliage. The world of horticultural oils is more complex so today we will limit our discussion to dormant oils which are applied in January and February here in Memphis.

Like other plant treatments this is one that seems to be almost too good to be true. It kills some of the most difficult insects to control, killing not just the target insect but also their eggs. However you should not apply this material indiscriminately as there are precautions to take. As good as it sounds there are limits to its effectiveness.

Insects effectively controlled by dormant oil spray;

·         Aphids that curl leaves in spring

·         Caterpillars that winter as eggs on the plant (leafrollers, tent caterpillars)

·         Mites that winter on the plant 

·         Scale Insects

·         Whitefly

Benefits of Dormant oil spray;

·         It kills specific pests and is rarely toxic to beneficial insects 

·         Applied in the dormant season to prevent pest outbreaks later on 

·         Dormant sprays are low risk to pollinators which are not yet active

·         Overall pesticide use is reduced by suppressing pest populations before they buildup

Precautions of Dormant oil spray;

·         Read and follow all label directions

·         Apply when temperatures will remain above 40°F. for 24 hours If spray just prior to temperatures dropping below freezing, the emulsion breaks down, causing the oil to adhere to the bark/leaves instead of the targeted insect.

·         Do not apply oils if plant tissues are wet or rain is likely.

·         Dormant oil can cause leaf burn and defoliation when applied to green tissue. Plants that tend to be sensitive to oils should not be sprayed these include; Cryptomeria, Junipers and cedars, Maples (Especially red Maple and Japanese maples) Hickory, Black Walnut, Beech and Redbud. Avoid getting spray on surrounding plants.

·         Oils cannot be combined with sulfur products or sprayed within 30 days of a sulfur application.

Instead of running for toxic chemicals at the first sight of an insect problem you can save time, money and effort by following the principles of Integrated Pest Management (IPM). This is a fancy term that means you frequently take a stroll through your garden with a cup of coffee or glass of wine to admire your garden and check your plants for signs of damaging insects or disease.  By catching problems early you can save yourself time, money and worry.

When you find something that looks like trouble the first step to control the problem is to consider if it could have been prevented.  Pests and disease will attack stressed and weakened plants.  If you have chronic problems with a plant it may be that it does not have the conditions it needs to thrive.  Compare what things the plant needs to thrive with the conditions in your garden.  Is the pH correct, does it receive too much or too little water or light, is the drainage adequate? Fixing issues in the plants environment can return your plant to vigor and allow it to fight off the insects or disease that have plagued your plant.  

If there are no cultural issues, you then work to correct any problems using the simplest methods first.  For example a few scale insects on your crapemyrtle can easily be removed by rubbing or pinching them with your fingers. A larger infestation may be removed with a sharp spray of water from a hose or the arrival of their natural predators. Should neither of these actions control the problem or your crapemyrtle is too large for these methods to be practical you may want to turn to a more aggressive intervention.  

In this situation Dormant Oil may be the logical next step in our IPM journey. Using this agent is targeting the specific pest you need to control, without impacting beneficial or benign insects.  Reducing the scale population may be all that is needed to return your plant to full health.  

However should you have a large specimen that is difficult to reach with sprays the next step would be a systemic control that targets sucking insects such as scale.  A product containing Imidacloprid should be applied to the soil March through early May according to label directions. It is taken up by the roots and carried throughout the plant so a scale insect feeding at the very tip of the plant will ingest the chemical and die.  In a different scenario you may need to move to the final step in the IPM process and use a non-specific pesticides as a last resort.  

Follow these IPM steps, stopping your efforts at the least aggressive intervention needed to fix any problem you are addressing.