What's the best way to get rid of scale

[Discus]

I've tried repeatedly scraping these things off plants with earbuds and alcohol, but they always come back; I've also tried permethrin (with marginal success). Have resorted to a systemic insecticide (imidacloprid, which is at the center of an "arghh, it might kill bees" scandal, so may or may not be currently available where you are), which seems to be doing the job now.

I've lost quite a few plants to sad, slow scale-induced decline.