Hi Chris,
I would suggest Merit Systemic Insecticide, wettable powder. It is a
bit pricey, about $50 for a 2 ounce bottle, but you only need to use
it once or twice a year. I used it twice one year and have not been
bothered with scale since. That was 3 years ago. In fact the produce
states not to use it more that 2 to 3 times a year. It takes about 3
months for the product to be distributed throughout the plant, so if
you don't see results right away just be patient.

Geoff
Austin, TX