Scale sounds and looks a lot like a plant disease, but scale are actually tiny parasitic insects. They adhere to the stems and branches of plants and feed off the plant’s sap. Scale look like bumps and it is easy to see how they could be mistaken for a disease. But it is important to know the difference if you want to treat and correct the problem.
Identifying Scale Insects
There are some 7,000 species of scale insects, so there is a good chance you will encounter them at some point, in your garden. Scale 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 colors that blend 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 - 1/8 inch. However, you will never see just one of them, which makes them hard to miss.
Scale insects are usually divided into 2 groups: soft scale and hard, or armored scale. Soft scale is covered with a protective waxy substance and is somewhat easier to kill than 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 will not reach 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 scale family.
How to Control Scale Insects
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 scale.
Because adult scale insects are so firmly attached to their host plant and covered in their armor coating, they can be almost impossible to exterminate. It is not uncommon for plants to simply be thrown out, rather than take all the steps necessary to try and eradicate the scale before they do not have time to move on to more plants. However, there are some control methods that have proved effective against scale.
Control measures are most effective during what is called their "crawler stage". They are called crawlers when they first hatch, because the nymphs have legs at this point and must crawl off to find their own spot on a plant, to attach and feed. That's when they can successfully be killed with pesticides. However, timing is everything and there is a very short window of opportunity. Here are some measures you can take.
Controlling Scale on Outdoor Plants
- Scale insects can overwinter as nymphs or eggs tucked away in tree bark. Spray recently infested plants with dormant oil, in late spring, just before the leaves unfurl. This will smother the scale before they have a chance to form their protective coating.
- If you catch the problem early, pruning infected branches is often the easiest and surest solution. Inspect the plant thoroughly, as well as any adjacent plants, to ensure you have gotten all infested stems.
- Scale are preyed on by beneficial insects like soldier beetles, lady beetles, and parasitic wasps. You can encourage them 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 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 by rubbing gently with a facial quality sponge 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.