Imagine a bug that lived its life inside its own little protective shelter, looking like little more than a bump on a log. Scale bugs are just that--to most gardeners, they are difficult to even identify as insects. Scale bugs infest tropical houseplants, where their distinct form helps them to resist your best attempts at thwarting them.
There are two types of scale insects--soft and armored. According to the Cornell University Insect Diagnostic Laboratory, soft scales affect houseplants more often. Both types of scales produce a shell that protects the entire insect and repels most attempts to control them. Soft scales form their shell from waxy secretions and are able to move later in life. Armored scales shed layers of skin that, when combined with wax, forms a hard and immobile dome under which the insect spends its life.
According to the Colorado State University Extension, brown soft scales are one of the most common to affect houseplants. They appear as small brown dots near leaf veins and stems. Homeowners often notice soft scale first because plant leaves become sticky with honeydew, a substance secreted by soft scales. Armored scales do not produce honeydew, so you will need to look for bumps on the stems or leaves. Scale insects are often colored and textured to help them blend in on a leaf.
Scale insects directly injure the plant by inserting their mouthparts into the stem or leaves, extracting nutrients and reducing plant vigor. The honeydew produced by soft scales also encourages secondary infections with sooty mold, which appears as black fuzz on the leaves and blocks the plant from receiving adequate light.
Female scales lay eggs under their protective covering, which hatch into larvae called crawlers. Crawlers emerge from their mother's shell and move to their own location--one of or the only times some scale insects move. Once scales find a place to settle down, they form their protective coverings and often don't move again. The crawlers are most susceptible to insecticide treatment but, as the Cornell University Insect Diagnostic Laboratory states, without seasonal changes in an indoor environment, generations can overlap and the emergence of crawlers can be hard to predict.
The protective covering on scale insects protects them from most insecticide treatments. The Colorado State University Extension recommends using horticultural oils to treat adult scales or removing the adult insects from the plant. Scales in the crawler stage are more susceptible to insecticides, although overlapping generations found on houseplants may mean that several months of treatment are necessary to eliminate the entire population.