I'm not a plant guy but when to a pesticide class where the guy talked about scales.
If its soft scale he suggested acephate or seven
If its armor scale he recommended safari, oil, distance, kontos, talus, or a few others
He also said that merit would have no impact. On armor scale because of the tough shell.
After going I have figured that pest ID is way over my head and there are too many pest!! I'm years away from being comfortable enough to offer tree and shrub.