Scales are unusual looking and many people do not at first recognize them as insects. Scales do not have a recognizable head or wings or legs. They have a characteristic roundish, flat or humped appearance. Scale eggs hatch into crawlers which move around until they settle down and begin feeding. Most mature into adults without moving from that spot. They can have several generations per year.
When plants are heavily infested with scales, leaves may look wilted, turn yellow, and drop prematurely. Scales sometimes curl leaves or cause deformed blemishes or discolored halos in fruit, leaves, or twigs. Bark infested with armored scales may crack and exude gum.
Scales are often well controlled by beneficial predators and parasites. If scales become too numerous, a well-timed and thorough spray using horticultural (narrow-range) oil or insecticidal soap applied when scale crawlers are active in spring to early summer should provide good control. Complete spray coverage of infested plants including the underside of leaves is needed. Thorough spray coverage is especially important when treating armored scales, as these scales are generally less susceptible to pesticides than soft scales.
Some scale species excrete honeydew, a sweet, sticky liquid which attracts ants and sooty mold. If the scale secretes honeydew then control of ants is important. Keep ants out of tree canopies by pruning branches or weeds that provide a bridge between buildings or the ground and by applying a sticky material (such as Tanglefoot) to trunks.
Alternatively, or as a supplement, place enclosed ant baits such as ant stakes near nests or on ant trails beneath plants. Baits act slowly over a period of hours to weeks but can be much more effective than sprays. Read more about control of ants.
Read more about control of scale insects.