I'm not sure what type of scale you have,but my prickly pears in SoCal would get covered with what I thought at first was cochineal (sp?), but was advised that it was a different scale entirely and don't remember which it was called (white, powdery looking, rather like mealy bug)...however, a strong spray of water to wash them off was all that I found necessary to control it.
edited to add that mine were very large,sturdy cactus.
