I've heard good things about horticultural oil --
Many people prefer to use homemade control of plant scale. Insecticidal soap is a safe and effective alternative to conventional insecticides. You can use bleach-free dishwashing liquid (1 1/2 teaspoons per quart of water) in place of commercial insecticide soaps. Homemade control of plant scale can also be achieved with oil spray. Mix 2 tablespoons of cooking oil and 2 tablespoons of baby shampoo in 1 gallon of water. This can also be mixed with 1 cup of alcohol to help penetrate the insect’s shell.
Read more at Gardening Know How: Scale Bug – How To Control Plant Scale
http://www.gardeningknowhow.co...
Make sure to test it on a small portion of the plant first so you don't kill it
