Gardening glossary
Scale
Scale insects (superfamily Coccoidea) are among the most disguised pests in cultivation because most of their adult life is spent immobile under a protective covering. Adult females do not look like insects at all — they resemble small brown, tan, white, or grey bumps roughly 1 to 5 mm across, glued to stems, leaf veins, and the undersides of leaves.
There are two broad groups. Armored scales (family Diaspididae) build a hard wax-and-shed-skin shield that separates from the insect's body underneath, so flipping the shell with a fingernail reveals a soft creature still on the leaf. Soft scales (family Coccidae) have a more rounded, often shiny or waxy covering that is part of the insect's body. Soft scales produce abundant honeydew; armored scales produce little or none, which is a useful diagnostic clue.
The mobile life stage is the crawler — a tiny six-legged nymph that hatches under the mother's shell, walks a short distance, settles, inserts its mouthparts, and begins to build its own covering. Once settled, scales rarely move again. This is why treatments aimed at the visible adults often fail: the wax shield repels contact sprays. Successful control either targets the crawler stage with insecticidal soap or horticultural oil every 7 to 10 days during a hatch, or uses systemic insecticides that move through the plant's vascular tissue and poison feeding scales from within.
On houseplants, look for scale on the undersides of leaves along the midrib, in leaf axils, and on stems of ficus, citrus, hoya, succulents, and ferns. Sticky residue or sooty mold on lower leaves often gives them away before the scales themselves are noticed.