How to Get Rid of Scale on Plants
Identifying and removing scale insects from indoor and outdoor plants
Scale on plants sounds like and looks like a plant disease, but "scale" refers to an infestation by any one of over 8,000 species of tiny, sap-sucking insects. Scale insects typically adhere to the stems, branches, and sometimes leaves of plants to feed on sap.
They have a shell-like bump appearance, which makes it easy to mistake them for a fungal or bacterial disease. Scale treatment on plants is much different than the treatment for plant diseases, so it is essential to know the difference.
Below, learn how to identify, combat, and control scale insects on indoor and outdoor plants.
The Spruce / Jiaqi Zhou
Before You Begin
Before taking steps to eliminate scale on plants, ensure it is scale and not a plant disease. It is often confused for fungal or bacterial infections, like leaf spots or blight.
Identifying Scale
To properly identify scale, know what to look for, such as identifying common varieties of scale insects, early warning signs, and indications of damage.
Types of Scale Insects
Scale insects are closely related to aphids and whiteflies. They vary significantly in color, shape, and size. They are often somewhat rounded, but not always. Different scale varieties can be white, black, orange, or a color that blends in with the plant's coloring, making them even more challenging to detect.
Scale insects are usually divided into soft scale and hard or armored scale. Soft scale is covered with a protective waxy substance and is somewhat easier to kill than hard or armored scale.
Armored scale secretes a hard shell over its body for protection from predators. The shell also makes it difficult to use a pesticide because it has trouble reaching the insect inside.
Different species of scale insects favor different plants. Euonymus shrubs are frequently infested by scale. Other common scale victims are magnolia and fruit trees.
A member of the soft or unarmored scale family, mealybugs are a widespread garden pest and somewhat larger than most scale insects, making them easier to identify as an insect rather than a disease.
Signs of Scale on Plants
Most scale insects are small, ranging from 1/16 to 1/8 inch long. However, you will never see just one of them, which makes them hard to miss.
Scale almost always appears in clusters. Unlike other insects, they are immobile once they lock themselves into place to pierce the plant and begin feeding on sap. If you see clusters of tiny shell-like bumps on the stems or leaves of a plant, it is likely scale.
The appearance of sooty mold on the plant is a sign of scale. The insects produce sugary honeydew when feeding on plants which attract fungal organisms that produce sooty mold in some scales, but not all. The blackish mold is one of the most visible indicators of scale.
Scale Damage
Along with allowing fungal diseases to thrive through their honeydew deposits, scale harms your plant by depleting it of sap, which hinders the plant’s ability to send nutrients through its entire body.
Scale also causes cellular damage to the plant, making it structurally weaker. If left untreated, the scale will damage and weaken your plants and most likely cause them to die.
When to Combat Scale
Scale insects are adept at protecting themselves at most life cycle stages. Scale eggs are laid under the female's body, so the protective outer coating of the mother insect shields them.
Control measures are most effective during the scale insect's "crawler stage"—the nymphs that appear soon after the eggs hatch. Now, the nymphs have legs and are actively crawling to find new spots to attach and feed.
This is the time when they can be effectively killed with pesticides. However, timing is everything, and there is a short window of opportunity.
Because adult scale insects are so firmly attached to their host plant and covered in their armor coating, they can be tricky to exterminate. Sometimes, throwing out infested plants is easier than taking all the steps necessary to try and eradicate the scale.
Quick removal can prevent the scale from migrating to surrounding plants. However, if this is impractical, several control methods have proven effective against scale.
What You'll Need
Equipment / Tools
- Pruners
- Cotton swabs or makeup sponges
- Spray bottle
- Hose-end sprayer
- Garden sprayer
Materials
- Rubbing alcohol
- Horticultural oil
- Insecticidal soap
- Beneficial insects
- Spray pesticide (organic or chemical)
- Neem oil
- Dish detergent
Instructions
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Controlling Scale on Outdoor Plants
Scale insects in the garden can be combated in several ways, the best of which involves prevention or removing infested plant material before the insects can spread. While you may not need to take all these steps, you may need to employ a combination to defeat a scale bug infestation.
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Prune
Pruning infected branches is often the easiest and surest solution if you catch the infestation while it is still light. Inspect the plant thoroughly and any adjacent plants to ensure you have removed all infested stems.
Do not compost infected plant material; it is best to bag it up and throw it in the trash or burn it safely.
The Spruce / Almar Creative
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Treat With Rubbing Alcohol
Rubbing alcohol can kill scale insects if the infestations are light. The best approach is to put the rubbing alcohol directly on the scale bugs using a cotton swab.
However, this can be pretty laborious in an outdoor garden, so you can also fill a garden sprayer or spray bottle with a mix containing one part rubbing alcohol to seven parts water.
Repeat every two to three days until the problem insects are eradicated.
The Spruce / Almar Creative
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Spray With Horticultural Oil
What Is Horticultural Oil?
Most horticultural oils are petroleum-based mineral oils, although some vegetable oils also fight pests, such as cottonseed oil and soybean oil. The oil is usually emulsified to make it easier to spray.
Spraying your plants with horticultural oil is effective in late spring, just before the leaves unfurl. Scale insects can overwinter as nymphs or eggs tucked away in tree bark.
Spray your plants at this point using a hose-end or garden sprayer filled with 2 to 5 ounces of oil per gallon of water. This application will smother the scale before the insects form their protective coating.
It's essential to treat the entire plant, as scale can be present on stems, the undersides of leaves, and along the base of the plant. The oil coats the scale insects and clogs their breathing pores, suffocating them.
The Spruce / Almar Creative
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Apply Insecticidal Soap
Insecticidal soaps can be used to kill scale at the larval stage, but they are not very effective once the insects are anchored and fed under their protective shells. Apply with a spray bottle or garden sprayer until all the leaves are dripping.
A mix of 1 ounce of soap to a gallon of water is generally a good rule, but follow the package directions to be sure.
Several applications will be necessary to catch all the larvae because these soaps don't last long in the elements, but these organic pesticides will not leave a dangerous environmental residue.
Warning
Only apply insecticidal soaps to well-hydrated plants, or you'll risk damaging them. Also, never use the product when the plants are in direct sun or above 90°F.
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Apply Neem Oil
Neem oil, or any pesticides containing azadirachtin, a key ingredient in neem oil, offers excellent protection against scale and kills the established insects—not just the larva.
Because neem oil won't mix easily with water, you'll want to add a bit of dish detergent as an emulsifier. A general guide is to use 1 to 2 teaspoons of detergent and 1 to 2 tablespoons of oil to a gallon of water. Apply with a garden sprayer.
Neem oil and other azadirachtin treatments are considered organic pesticides and are not toxic to honey bees and most other beneficial insects. Other plant-based pesticides can also be successful.
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Use Beneficial Insects
Beneficial insects such as soldier beetles, lady beetles, and parasitic wasps can be an important and effective control for scale insects.
You can encourage these natural predators by providing them with food and shelter. You can also purchase beneficial insects by mail to release in your garden.
The Spruce / Phoebe Cheong
Warning
Synthetic chemical pesticides should be the last resort in the war against scale. While there can be some success in using systemic insecticides that include compounds known as neonicotinoids (acetamiprid, dinotefuran, imidacloprid, and thiamethoxam), these pesticides are now known to be a severe problem for honey bees and other pollinators. Don't reach for a chemical pesticide unless all other methods have failed.
Controlling Scale on Indoor Plants
Since no natural predators live indoors, scale insects will spread faster on houseplants than outdoors in the garden. You must diligently control or remove scale when it infests indoor plants.
If you catch the problem early enough, pruning out the infested stems could alleviate the problems. Keep a close eye on the plant for several weeks to ensure no new scale appears. Dispose of the pruned stems immediately.
Remove existing scale on houseplants by rubbing gently with a facial-quality sponge or cotton swab dipped in rubbing alcohol. The alcohol alone should kill the scale, but the dead insects will remain on your plants, making it difficult for you to scout for new infestations.
The small facial sponges in the cosmetic aisle are abrasive yet soft enough to use without scraping the plant stems. Be sure to buy plain sponges without cleanser or lotion in them. As always, test on a small area first since some plants are more sensitive than others.
The Spruce / Phoebe Cheong
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Can plants survive scale?
Plants can survive scale and death from an infestation is rare. However, a scale infestation can weaken plants or trees.
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Can I use vinegar to remove scale on plants?
Use vinegar to remove scale insects up to a degree, but it may not be completely effective.
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Should I throw away a plant with scale?
You shouldn't need to dispose of your plant. Quarantine it by keeping it at least 3 feet away from other plants while treating it. If it's badly infested, prune off infested branches. If your plant looks dead or unrecoverable, discard it in the garbage. Do not compost it.
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How contagious is scale on plants?
Scale is very contagious and spreads quickly. Isolate any plants with scale from your other plants, if possible.