Scale sounds and looks a lot like a plant disease, but the term actually refers to infestation by any one of more than 7,000 species of tiny sap-sucking insects. Scale insects typically adhere to the stems, branches, and sometimes the leaves of plants to feed on sap, and they have a shell-like bump appearance, which sometimes causes them to be mistaken for a fungal or bacterial disease. But the treatment of scale is much different than the treatment for plant diseases, so it is important to know the difference if you want to treat and correct the problem.
Because there is such a vast number of different scale insect species, there is a good chance you will encounter them in your garden or on your houseplants at some point. Scale insects vary greatly in color, shape, and size. They are often somewhat rounded, but not always. Different varieties of scale can be white, black, orange, or a color that blends in with the plant's coloring, making them even more difficult to detect.
Most scale insects are very small, usually ranging in size 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, there is a good chance you are looking at a scale. One of the primary ways gardeners realize they have an issue with scale is by the appearance of sooty mold on the plant. Scale insects produce a sugary honeydew when feeding on the 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 insects are usually divided into two groups: soft scale, and hard or armored scale. Soft scale is covered with a protective waxy substance and is somewhat easier to kill than hard/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. Plants frequently infested with scale include Euonymous, magnolia, and fruit trees and shrubs. Mealybugs, a widespread garden pest, are also part of the soft or unarmored scale family. But mealybugs are somewhat larger than most scale insects, which makes them easier to identify as an insect rather than disease.
Timing Your Treatment
Scale insects are very adept at protecting themselves at most stages of their life cycle. Scale eggs are laid under the female’s body, so they are shielded by the protective outer coating of the mother insect.
Control measures are most effective during what is called the scale insect's "crawler stage"—the nymphs that appear soon after the eggs hatch. At this point, 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 very 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, it is easier to simply throw out infested plants rather than take all the steps necessary to try and eradicate the scale. Quick removal can prevent the scale from migrating to surrounding plants. But if this is not practical, then are several control methods that have proved effective against scale.
- Working time: Less than one hour per plant
- Total time: Inspection and treatment are both necessary every few weeks.
- Material cost: Insecticidal soap costs $5 to $10 for a 24-ounce spray bottle; neem oil spray costs about $12 for a 24-ounce bottle; a bag of 1500 live lady beetles costs less $10 or less.
What You'll Need
Scale is a difficult problem to combat, and you may need to employ all of these materials if infestations are frequent and widespread.
- Cotton swabs or makeup sponges
- Rubbing alcohol
- Horticultural oil (neem oil)
- Insecticidal soap
- Beneficial insects (optional)
- Spray pesticide (organic or chemical)
Controlling Scale on Outdoor Plants
Scale insects in the garden can be combatted in a number of ways, the best of which involves prevention or removing infested plant material before the insects can spread.
- Spraying 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. This application will smother the scale before the insects have a chance to form their protective coating. It's important 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.
- Pruning infected branches is often the easiest and surest solution if you catch the infestation while it is still light. Inspect the plant thoroughly, as well as any adjacent plants, to ensure you have removed all infested stems. Do not compost infected plant material.
- Rubbing alcohol can kill scale insects if the infestations are light. This can be quite laborious in an outdoor garden, however.
- Insecticidal soaps can be used to kill scale at the larval stage, but they are not very effective once the insects are anchored and feeding under their protective shells. Several applications will be necessary to catch all the larvae, but these organic pesticides will not leave a dangerous environmental residue.
- Pesticides containing azadirachtin, a key ingredient in neem oil, offer excellent protection against scale, and also will kill the established insects. These are regarded as organic pesticides and are not toxic to honey bees and most other beneficial insects. Other plant-based pesticides can also be successful.
- Synthetic chemical pesticides should be the very 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 serious problem for honey bees and other pollinators. Don't reach for a chemical pesticide unless all other methods have failed.
- Beneficial insects such as soldier beetles, lady beetles, and parasitic wasp 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.
Controlling Scale on Indoor Plants
Since there are no natural predators indoors, scale insects will spread even faster than they do outdoors. You will need to be extremely diligent about controlling or removing 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 for several weeks to make sure no new scale appear on the plant. 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 and make it difficult for you to scout for new infestations. The small facial sponges, found 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.