Applying Dormant Oil
Scale insects, mites, aphids, and other pests can overwinter on certain fruit trees and ornamental plants as eggs, immature stages, or adults. When pest populations become heavy, their feeding can weaken plants and affect overall plant health.
One tool for managing certain overwintering pests is horticultural oil. When applied correctly and at the appropriate time, dormant or delayed-dormant oil can help reduce populations before spring growth begins.
Aphids on a shrub
Scale insects on a fruit tree
How Does Horticultural Oil Work?
Horticultural oils work primarily by coating and suffocating susceptible insects, mites, and eggs that are contacted by the spray. Because the oil must physically contact the pest to work, thorough coverage is especially important.
Oils can be useful as part of an integrated pest-management program and generally have a relatively short residual effect compared with many broad-spectrum insecticides. However, they can still affect beneficial insects that are directly contacted by the spray, so apply them carefully and only when needed.
When to Apply Dormant Oil
Don’t choose an application date based on the calendar alone. Follow the particular horticultural oil’s label for the plant, pest, temperature range, and growth stage you’re treating.
Dormant applications are generally made before active spring growth, while delayed-dormant applications may be recommended for certain pests and plants as buds begin to swell. The correct timing depends on both the plant and the pest you’re trying to control.
Choose calm, dry weather and avoid spraying immediately before freezing temperatures, rain, or other conditions prohibited by the product label. Temperature restrictions vary among horticultural oil products, so don’t rely on a universal 40°F rule.
How to Apply Horticultural Oil
• Read the label first. Make sure the product is labeled for both the plant and pest you’re treating, and follow all mixing, temperature, safety, and application instructions.
• Protect nearby plants if the label indicates they may be sensitive to the product. Some plants and tender new growth can be injured by horticultural oils.
• If using a concentrate, mix it according to the label directions. More oil is not better and can increase the risk of plant injury.
• Thoroughly cover the branches, twigs, bark, and other areas where the targeted pest is present. Work around the plant so the spray reaches cracks, crevices, and other hiding places.
• Avoid applying oil to drought-stressed or otherwise severely stressed plants unless the product label specifically allows it.
What Happens to Scale After Treatment?
Don’t expect every visible sign of a scale infestation to disappear immediately after a successful treatment. Dead scale coverings may remain attached to branches and bark for some time.
If the infestation produced honeydew and sooty mold, the dark coating may also remain visible after the insects have been controlled. Sooty mold grows on the sugary honeydew produced by certain sap-feeding insects, and the old coating gradually weathers away once the source of honeydew has been reduced.
Check Before You Spray Again
Before automatically making another application, inspect the plant to determine whether live pests are still present. Scale insects can be difficult to evaluate because their protective coverings often remain attached even after the insect underneath has died.
You can gently lift or scrape away several scale coverings and examine what is underneath. Dry or shriveled insects generally indicate dead scale, while soft or moist insects may still be alive.
If live scale remains, identify the type of scale if possible and check the product label for appropriate retreatment timing. Some scale insects are most effectively controlled during the vulnerable crawler stage, so additional treatment should be based on the pest’s life cycle rather than simply spraying again after an arbitrary number of days.
Remember: Identify the Pest First!
Horticultural oil can be an excellent tool for controlling certain scale insects, mites, aphids, and other soft-bodied pests, but it isn’t necessary for every plant every winter. Inspect your plants, identify the problem, and use dormant or horticultural oil when the pest, plant, and product label indicate it’s appropriate.