It is always a good idea to test spray a small area, maybe in the back of the tree to see how it reacts. Horticultural oil that you buy works very well on scale as well. However, it is a pesticide and although the toxicity is low, there are still health hazards. What I like about the home made stuff is that in my last 7-8 years of dealing with scale, I have noticed that the homemade recipe works just as well as the store bought. Also, when sprayed, a tree can be near pets, in a garage, green house, house etc without having to worry about it comiing in contact with people or animals.
Also, as Rock and I originally stated, the main focus is picking them off... There are different kinds of scale.. Oyster shell scale is very hard to kill with pesticides because it does not penetrate. However, white juniper scale and soft bodied other forms can be killed by it.
Scale eggs are carried by wind and also ants. If you have an ant problem, that might be adding to the scale issue.
p.s. The oil I used to use before I discovered the home made stuff was Bonide brand All seasons Horticultural Oil concentrate. It is not restricted to dormancy or other limits. However, always read the labels thouroughly. As I said, this product worked well, but the homemade stuff has less restrictions.
Rob