spray applications will only control the nymph stage of the scale. They usualy ffed on leaves. This will help for next year, but not much for the current years suckers. The eggs are laid under this years females shell, so dormant sprays wont help. Merit is a growth regulator so that dont work either.
I'am having a problem too with infestation beynd they economic threshold. I have been doing sprays like you and foliar/soil drench apps of organic suppliments that have elements such as iron magnesium and to keep the tips from being sucke dry. So to speak.
I have finaly found someone who will do an application of metasystex (sp?). a systemic used as a soil injection that is labled for the adult stage of the pest. A number of companies have told me that it works real good, but eats their pumps and hoses up so they find they cannot charge enough to justify the cost. It is rather acidic.
Donot use a high N fertilizer (the first number in the 30-30-10) for a few years. Use liquid fish or seaweed emultion to get (do I sound like a broke record, yall?) maintain moist soil conditions, not wet. and call around for someone to put this "hot" product in your ground, or just spray regularly with sevin or something this year and next.