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There are 4 different species of scales that attack arbs in your area. You should bring a sample of the arbs in question with some scales to your local cooperative extension office to find out which scales you have.....Then you will be able to go to the next step which is the life cycle of the pest. The best time to spray is when the scales are in the crawler stage as tideman has discribed. Your cooperative extension office will be able to tell you when the scale you have to deal with is in the crawler stage with a 50 % hatch. When you get ready to make a control app. you can look for crawlers with a 10x or 20x hand lens to confirm crawler activity then spray with a systemic insecticide like ORTHENE. I would not use merit because spruce spider mites will buildup populations as a result.
Plan on a 2% dormant oil spray in the next 2-3 weeks or when night time temps are above 40F to suffocate most of the scales on the arbs. This will get most but not all scales.
If you can post a pic of the scale on the arbs I can identify for you and give you aproxx timing to scout arbs for crawlers and make apps.