How to Treat Scales on Plants

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Two scale insects on a green leaf
Two scale insects on a green leaf (Image: TrichopCMU/iStock/Getty Images)

The first inkling that scale insects have attacked your plants may come with the discovery of small tan bumps along plant stems or a trail on honeydew-gorged ants making their way through your garden. Scales have small piercing mouthparts that penetrate plant tissues and suck out plant juices. Many different scale insect species affect the bark, fruit and leaves of plants, but they fall into two main categories. Treatment depends on the type of scale involved and the stage of the scales' life.

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Understanding Your Enemy

With few exceptions, scale insects are either armored or soft. Close examination tells which kind they are. Adult armored scales have a hard, shieldlike plate over their bodies. The 1/8-inch-long insects grow the covering as they mature. Not attached to the scale's body, the armor can be lifted off. Even so, it provides a protective covering that makes it harder to treat armored scales. Soft scales are slightly larger, near 1/4 inch, and more rounded. Their waxy coverings are part of their bodies and cannot be removed. Only soft scales excrete honeydew. Armored scales produce several generations each year, while soft scales produce just one.

Intervening Through Culture

Scale species are often tied to a single plant species. Keeping plants at the peak of health reduces scale attacks and damage. Proper watering is especially important because stressed plants attract scale. If your plants begin to yellow and wither or have distorted leaves or buds, check the stems and leaves for scale insects, their crawling nymphs and eggs. Most armored scales spend their entire lifespan in one place. Soft scales move slowly, but and rarely. Minor scale infestations often don't require any treatment. Remove the scales by hand when you catch them early, or prune out heavily infested stems. Use sharp bypass pruners and wipe the blades with household disinfectant before and after each cut.

Enhancing Natural Predators

Scale problems often start when natural scale predators are inhibited in some way. Dusty conditions or using garden chemicals in the area can kill beneficial insects. Honeydew attracts ants that feed on the sugary liquid and interfere with scales' natural enemies. With no predators, scale populations grow. Eliminating ants by placing ready-to-use ant bait stations near their trails enhances natural parasites and predators. Releasing commercially available beneficial insects helps, too. Small parasitic wasps are effective in combating scale. The wasps lay eggs in scales, and hatchlings feed on their host. Exit holes can be seen on dead scales. Lady beetles and lacewings feed on scale eggs and crawling nymphs.

Using Chemical Controls

When other methods fail, ultra-fine horticultural oil treats both armored and soft scales. Hatching scale nymphs are most vulnerable. Treat scale during the dormant season or in late winter and early spring when nymphs hatch. Fewere leaves means less surface to cover and fewer hiding spots for scales. Mix 5 to 7 tablespoons of horticultural oil concentrate with 1 gallon of water in a garden sprayer. Spray to completely cover stems and both leaf sides. Thorough coverage is essential with adult and armored scales. Only spray when temperatures are between 32 and 90 degrees Fahrenheit, and never spray drought-stressed plants. Wear gloves, protective clothing and safety goggles when you spray. Repeat every seven to 10 days, or as recommended on the horticultural oil label.

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