If you see bumpy roundish spots on twigs, branches or leavs, your tree probably has scale insects. Scale insects can sometimes be serious pests but plants are not usually harmed by a few scales or even a high population of certain species. Scales insert a tiny strawlike mouthpart into plants and suck fluids. Scales can occur on bark, fruit, or leaves.
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.