Answer: Scale is an insect and can be treated when in the soft "crawler" stage with insecticidal soap used according to the label instructions. Be sure to treat all surfaces and crevices as well as stems because they are sometimes difficult to see even in bright light. Gently scraping them off with a fingernail is another option but is somewhat time consuming on a large or intricate plant and you must inspect frequently if you hope to catch them all. It is difficult to control, as you are seeing. Sometimes if a plant is badly infested and not terribly rare or valuable it is worth considering getting rid of it to avoid spreading the problem to neighboring plants.
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