What to do? Kill. Parasites are a real enemy apart from being disgusting creatures that are disfiguring your lovely trees and perennials. Because scale protects itself by exuding a thin secretion over its body they can be difficult to eradicate with ordinary pesticides. Isolate the plant, scrape off the motherships and the babies with a butter knife, then dip it in a kill solution of your choice so that you don't infect the rest of the garden (vinegar, ammonia etc). Spray the entire plant with Neem oil. Go ahead and don't be shy. Spray the undersides of all leaves, the stems and any new foliage. Agricultural oils will suffocate any remaining hold outs on your plant.
Check your plant regularly and spray regularly until you feel confident that your plant is safe from nature's little Dracula.