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Scale Insects: How to Get Rid of Scale and Mealybugs | The Old Farmer's Almanac

Scale Insects

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How to Get Rid Of Scale Insects and Mealybugs

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Scale insects and mealybugs are tiny, sap-sucking pests that can spread rapidly. You may have seen scale insects on leaves and stems but never noticed them! Inconspicuous, well-camouflaged, and small, they don’t attract much attention, but they can be a significant pest. Here’s how to get rid of scale and mealybugs on your plants!

What are Scale Insects?

Scale insects are tiny bugs that vary widely in appearance and look more like a blemish on a plant’s foliage than an insect. The adult females are immobile, and the nymphs, called crawlers, don’t move far from where they hatched. Only the males are genuinely mobile, flying around to find a mate. 

Although highly variable, scale insects share one common characteristic: They are covered or protected by a waxy substance that looks much like the scales on a fish. This protective covering makes them more resistant to standard chemical control methods.

Scale insects have piercing-sucking mouthparts that are used to feed on the sap of plants. While a few scale insects won’t cause much harm, infestations can significantly weaken a tree or plant, making it susceptible to other pests, diseases, and stresses. 

Some scale insects create a sticky substance called honeydew, which can cause the growth of black sooty mold on the leaves and block the sunlight needed for photosynthesis.

Scale insects are divided into two main categories: soft-scaled and armored. 

mealybugs
Mealybugs. Photo by Protasov AN/Shutterstock

Soft Scales

More common on indoor plants, soft scale insects often inhabit the underside of leaves or stems. After hatching, nymphs will crawl to a new location where they will settle down for the rest of their lives. 

Mealybugs are one type of soft-scaled scale insect. They secrete a powdery, waxy coating that is often white or light colored. They may have a hairy or cottony appearance as well.

Younger scale insects are typically lighter in color and darken as they mature. Outdoors, the honeydew they create can make a sticky mess on the ground under heavily infested trees. 

Scale insect pest on plant leaf. Photo by Cherchai Chaivimol/Shutterstock
Armored scale. Photo by Cherchai Chaivimol/Shutterstock

Armored Scales

The smaller armored scales have a more rigid, waxy shell-like covering. They may be hard to spot if their color closely matches the plant. 

Hard scales can cause more damage to their host than soft scales and may even form a thick crust of insects that feed heavily on the plant. They do not create honeydew like soft scale insects and typically infest trees and shrubs.

Check out this overview of scale insects by Iowa State University Extension and this pamphlet from the University of California IPM program for more information. 

Identification

These tiny pests are difficult to see. They can be as small as ⅛ of an inch. They often look like a bump, growth, or bud scar. Some species may look like mold or crust. Once you recognize them, though, you may spot more of them around.

Look for these signs of scale:

  • Yellowing leaves. The plant may look sickly and wilted.
  • Leaves browning and falling off.
  • Drops of honeydew on the leaves or even collecting under the plant. It may be accompanied by black sooty mold on leaves.
  • Bees, ants, and wasps that are attracted to the honeydew. 
  • Scale insects can be small and look like everyday bumps or imperfections in the bark. Try a magnifying glass to take a closer look. 
Control and Prevention

If you have detected scale insects that are still active, it’s time to take action. A few options are available and depend partly on whether your problem is inside or outdoors.

Prevention

Scale insects can come from many sources, and eliminating them is impractical. Your best bet is healthy soil to keep your plants vigorous and some good old-fashioned monitoring to catch them early. 

Keep an eye on your houseplants and spend time walking your garden and yard. When you have a good feel for what “normal” is, you can more easily spot a change. Proper monitoring is an integral part of integrated pest management.

  • Investigate yellowing leaves promptly.
  • Outdoors, minimize the use of pesticides that may harm populations of beneficial, predatory insects like ladybugs. 
  • Dispose of heavily infected plant material to limit new generations of scale insects.
  • Closely inspect new plants when purchased, especially under leaves and between stems. Quarantine away from other plants if in doubt.

Indoor Scale Control & Treatment

A severely damaged plant may need to be disposed of, but less infested plants can be given some TLC.

Use a Toothbrush

Grab a soft-bristled toothbrush and some soapy water and start doing what your dentist taught you, only to your plant. A gentle brushing will often remove many adults and most crawlers. Moving your plant to the sink or shower may be less messy. 

Try to avoid over-wetting the soil, and use warm soapy water. You can also use a 70% solution of isopropyl alcohol and cotton swabs, dabbing them directly on the insects. Several treatments may be necessary to finally get them all. 

Brushing will also clean up some of the messy honeydew. 

Insecticidal Soaps and Horticultural Oils

Insecticidal soaps and horticultural oils are minimally effective against adult scale due to their waxy coatings. However, they are effective for controlling the crawlers (young scale) and thereby limiting the next generation. These types of treatments can often be found a local garden center or supply store. Apply to the undersides of leaves and stems, and repeat weekly for several weeks.

Outdoor Scale Control & Treatment

Removal

An infested tree or shrub may be pruned, and the infected branches removed and destroyed. Scale insects are not very mobile, so mechanical methods can work. Infected branches of larger garden plants can be trimmed off as well. 

Don’t throw them in the compost! Scale insects can overwinter as eggs under the shells. 

Insecticidal Soaps and Horticultural Oils

Insecticidal soaps and horticultural oils can control crawlers in the garden, orchard, or yard. Neem oil is also effective. Apply either solution weekly for at least 3 weeks to get control. Be cautious and do not overuse. Some of these chemicals can harm bees and other beneficial insects as well, so avoid spraying around flowers or when beneficial insects are spotted. 

Dead scale insects will not fall off the plant. To check if they are dead or alive, rub them with your thumbnail. Dead scale insects will be dehydrated and flake off. Live ones will smear.

Mix your Neem oil or insecticidal soaps and oils in a pump sprayer for a more practical application. Try to avoid spraying on drought-stressed plants, as it can worsen their condition. 

Encourage Beneficial Insects

Parasitic wasps, lacewings, ladybugs, and other predators can keep scale insects under control. If you have a large garden or orchard, these can be your best line of defense. Parasitic wasps will lay eggs in the scale insect; their larvae eat it before emerging. Ladybugs have a voracious appetite and will eat hundreds of scale insects, aphids, and other pests. 

You can encourage beneficial insects by planting pollinator patches or strips to provide habitat and food when there are insufficient levels of prey insects. If they can’t find food, they may relocate and leave you on the battlefield alone!

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