Scale, common on backyard trees

Common on backyard trees, ornamental shrubs, greenhouse plants and houseplants, over 1,000 species of scale insects exist in North America. They are such oddly shaped and immobile pests that they often resemble shell-like bumps rather than insects.

Scale insects vary dramatically in appearance; from very small organisms (1–2 mm) that grow beneath wax covers (some shaped like oyster shells, others like mussel shells), to shiny pearl-like objects (about 5 mm), to creatures covered with mealy wax.

Adult female scales are almost always immobile (aside from mealybugs) and permanently attached to the plant they have parasitized. They secrete a waxy coating for defense; this coating causes them to resemble reptilian scales or fish scales, hence their common name.
The first instars of most species of scale insects emerge from the egg with functional legs and are informally called “crawlers”. They immediately crawl around in search of a favorable spot to settle down and feed. In some species they delay settling down either until they are starving, or until they have been blown away by wind onto what presumably is another plant, where they may establish a colony separate from the parent.

There are many variations on such themes, such as scale insects that are associated with species of ants that act as herders and carry the young ones to favorable protected sites to feed. In either case, many such species of crawlers, when they change their skins, lose the use of their legs if they are female, and stay put for life. Only the males retain their legs and use them in seeking females for mating.

In many cases, heavy infestations build up unnoticed before plants begin to show damage.

Large populations may result in poor growth, reduced vigor and chlorotic (yellowed) leaves. If left unchecked, an infested host may become so weak that it dies.