This is common called Cottony Cushion Scale. It is a form of scale insect. These insect have sucking mouth parts and feed on the plants vascular tissue or sap. There are different methods that you use to control these insects. The simplest methods is using a high pressure water hose attachment to try to knock them off the plant. If that is impractical or ineffective, then trying an insecticidal soap can potentially work. There are systemic chemicals that can be drenched around the base of the tree. The roots of the tree absorb the chemical and kill the insects. Bayer Advanced and Greenlight sell products are your local garden center that can treat this type of problem. These products typically are called "Tree and Shrub Insect Control" . Please review the following links for more information on this insect and it's control.

http://www.ces.ncsu.edu/depts/ent/notes/O&T/shrubs/note51/note51.html

http://www.ipm.ucdavis.edu/PMG/r107301611.html