Scale insects are a difficult problem anywhere, because the bugs are well-defended with their shieldlike backs. They suck juices from plant cells and look innocent-like small raised bumps on plant stems and leaves. Insecticides are effective only when the young larvae (crawlers) are moving about the plants. To monitor crawlers, place pieces of double-sided tape on your plants; check them weekly for the presence of tiny beige dots, or tap infested twigs over a piece of white paper and look for moving specks. When the crawlers are active, spray the plants weekly with any all-purpose insecticide. Adult scale insects are a more serious challenge. On seriously infested plants, spray monthly with light horticultural oil. In spring and autumn, when temperatures are mild, occasionally use insecticidal soap instead of the oil. Meanwhile, douse the soil around an infested evergreen with a product that contains imidacloprid. The plants take up the chemical through their roots, and the scale insects die as they continue to feed. This chemical is much less toxic than older systemic pesticides, but it should be used only on ornamental plants.
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