Caren White is a Master Gardener and instructor at Home Gardeners School. She has been associated with Rutgers Gardens for over a decade.
What Is Scale?
Scale is actually a catch-all term for a group of insects that are covered with either a soft or hard shell that parasitize your plants. They pierce the plant tissue and feed on the sap, literally sucking the life out of the plants. They attack many different plants, both outdoor plants and houseplants. Some are specialized, only feeding on one species of plants. A good example is the euonymus scale, which only feeds on euonymus plant species.
There are about 8,000 different species of scale insects. Not all of them are bad. The cochineal group is the source of red dye for fabrics and food. The lac group is the source for shellac.
All scale insects are tiny, ranging in size from 1/8 to 1/16 of an inch. They come in many colors, including white, yellow, orange, brown and black. They are all covered with a shell that, depending on the species, is either a soft wax, known as soft scale, or a hard covering, known as armored scale.
With the exception of mealy bugs, the females are immobile. They attach to the plant and don’t move for the rest of their lives. They lay and hatch their eggs underneath them so that their shell can protect the eggs from predators.
When the eggs hatch, the hatchlings are known as crawlers. This is the only time that the females are mobile. The crawlers have legs which allows them to move to other parts of the plant to attach and feed, or, in some species, they move away from the female and then the wind blows them on to another plants where they attach and feed. If they are female, they lose their legs as they molt.
Male crawlers retain their legs so that they can seek out females for mating. Some males even have wings so that they can fly to other plants seeking females. Male scale insects do not feed. They live very brief lives of only a day.
Their miniscule size and impenetrable shells make scale insects hard to spot and even harder to kill. Here are a few suggestions for getting rid of them.
Use Systemic Insecticides
If you are not an organic gardener and don’t mind using insecticides, systemic insecticides are a good option. Systemic insecticides are sprayed on to the plants themselves or the soil surrounding them where it is taken up by the roots. The insecticides are absorbed by the plants and eventually reach the sap and poison it. When the scale insects feed on the sap, they are ingesting the insecticide and die.
Systemic insecticides are only recommended for houseplants and ornamental plants. They should never be used in vegetables, berries or fruit which are eaten by humans. The insecticide is toxic to humans as well as scale.
Use Horticultural Oils
Another solution which, depending on the oil used, can be organic, is to use horticultural oils which can be purchased at your local nursery. The oils are sprayed on the plants and hence on the scale insects. It doesn’t poison the insects because it doesn’t penetrate their shells. Instead, it smothers the insects by clogging their breathing pores.
A good horticultural oil, especially for use on edibles, is neem oil which is produced from the seed of the neem tree. It is organic, biodegradable and non-toxic to humans, animals and birds. After smothering the scale, it breaks down after exposure to rain or the ultra-violet rays of the sun.
Use Insecticidal Soap
Insecticidal soaps work the same as the horticultural oils, smothering the insects. It can also dissolve their hard shells. They are non-toxic for humans, animals and birds. You can purchase commercial insecticidal soaps at your local nursery or make your own.
If you decide to make your own insecticidal soap, it is important that you use the correct kind of soap. Do not use dry detergents formulated for washing clothes or dry dishwasher detergents. They are too harsh for your plants. Also avoid the use of dish detergents which have degreasing ingredients such as Dawn. You need to use pure soaps like Castile soap or natural soaps that contain fatty acids. Use 1 tablespoon of soap in 1 quart of water. For larger quantities, use 5 tablespoons of soap in 1 gallon of water. Apply using a spray bottle.
Pick Them Off the Plants
Houseplants can also be susceptible to scale and because they are smaller and in a confined environment, the scale insects can quickly overwhelm them. You need to be especially careful if you set your houseplants outdoors during the summer where they can become infected. When you bring them indoors in the fall, they can spread the insects to other plants very quickly. You should quarantine any plants that have been outdoors for 30 days to prevent the spread of scale.
If you see scale on any of your houseplants and the infection is confined to a single branch, you can cut off that branch and dispose of it. If the infection is not too severe, you can use horticultural oil spray or insecticidal soap spray just as you would your outdoor plants. If an entire plant is infected, your best bet would be to dispose of the entire plant.
You can also remove the insects manually by rubbing them off the infected leaves and stems with your fingers. Wear rubber gloves if you don’t want to touch the insects.
Rubbing alcohol dissolves the scale insects' shells which kills them. You can apply the alcohol using a cotton swab or a cosmetic sponge. A cosmetic sponge is softer than the sponges sold for household use and won’t injure your plants as a coarser household or scrubber sponge would. You will need to treat your houseplants for up to four weeks to ensure complete removal of all the insects.
Questions & Answers
Question: Which systematic insecticide is used to get rid of scale?
Answer: Systemic insecticides that include either Acephate or Imidacloprid can be used.
Question: Is scale harmful to be around for people or dogs?
Answer: Scale are insects that only infest plants. They will not bother humans or pets, just plants.
© 2018 Caren White
Caren White (author) on March 27, 2018:
Thanks Redelf! Insecticidal soap is a great defense for indoor and outdoor plants.
RedElf from Canada on March 27, 2018:
Interesting article. Lots of good information here. One of our biggest outdoor pests seems to be some kind of aphid that loves the very young poplar saplings. But I found washing the leaves with the soap you describe works for them too. Once I can't reach the leaves, the trees are usually big enough to fend for themselves :)