I get the scale on citrus trees, sapote trees and olive trees. To kill the scale I mix a Tablespoon of the green Palmolive dish soap with a quart of water in a spray bottle. I like the dish soap spray since I can get my hands in there to move branches around to spray everywhere. It works very good, the scale dries up in a week and dies.
I mention the "green" Palmolive because one time I used the pink Vitamin E enhanced Palmolive (since that was what was on the kitchen counter next to the sink) and all the leaves fell off the sapote tree in two weeks.
To prevent a massive scale infestation you need to stop the ants from climbing the tree who then protect, groom, distribute, breed, tend and milk the scale for their Honeydew. Just like tiny dairy farmers tending their cattle! Use Tanglefoot painted around the trunk, but NOT ON the trunk since it will kill the bark. Wrap the trunk with a strip of plastic ( I use the plastic yellow CAUTION tape, if you are a cop use CRIME SCENE tape) to protect the bark. Use spackling or some kind of putty to fill the rough bark so the ants cannot crawl under your tape, then put the tape around the trunk over the spackling. Never put duct-tape on the bark, the glue will damage the bark which will take years to heal (I tried duct-tape once for grafting. Big mistake.). Then apply the Tanglefoot on top of the plastic.
Once the ants are no longer in your tree then the predator bugs will come in and eat the pest bugs like scale and aphids.
