Answer: I don't think a few mites will cause the bronzing on your arborvitae. Mites like dry, dusty places and you can deter them by periodically hosing off your arborvitaes, directing the spray of water in towards the center of the plants. The bronzing is probably a normal reaction to cooler night air. Most will turn in the winter time but then green up again in the spring. I don't think it is anything abnormal but if you'd like you can clip a little of the foliage and take it into a Master Gardener Clinic or into King County Cooperative Extension (Smith Bldg.) in Seattle. They can positively diagnose the problem for you.
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