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How to Treat Scale Insects on Shrubs

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Scale insects feed on shrubs’ sap using their long, piercing mouthparts, causing twig and branch dieback and sometimes killing the plants. Scale insects also secrete a sticky liquid called honeydew, which promotes the fungal disease black sooty mold. Scale insects are difficult to get rid of, as they’re resistant to insecticides when they’re adults and can be controlled by chemicals only when they’re in the “crawler” stage. You can control scale infestations on your shrubs by utilizing natural scale predators, by spraying the shrubs with horticultural oils, by applying insecticidal soaps or by spraying the shrubs with approved insecticides.

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Inspect your shrubs for scale crawlers. Hold a piece of paper beneath the branches and shake the limbs to see crawling scale insects falling off. Scale insects are tiny, so you may require a magnifying glass to study them better.

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Introduce natural scale predators, such as ladybugs and tiny parasitic wasps. You can release the parasitic wasps or ladybugs onto your shrubs, and they’ll feed on the scale insects. Release the natural predators at the time of day and under the conditions recommended by the company from which you purchase them.

Spray your shrubs with horticultural oil, or paraffinic oil, after the scale crawlers hatch. Spray the oil generously over the shrub to coat the entire plant.

Apply an insecticidal soap to the shrubs’ leaves when the scale insects are in the crawler stage. Repeat the application about 10 days later.

Treat your shrubs with an approved insecticidal spray, such as carbaryl, acephate, malathion or chlorpyrifos. Ensure that the insecticide is approved for the scale insects you have and the types of shrubs that are infested. Apply the insecticide when the scale insects are in the crawler stage.

Tip

Spray your shrubs with dormant horticultural oils before bud break in early spring, usually in March or April. The dormant oils will kill the over-wintering scale insects on the shrubs.

Warning

Always follow the instructions on the label exactly when you’re applying horticultural oils or insecticides. Wear gloves, eye protection and a respiratory mask when handling or spraying these chemicals.

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