Answer: Browning foliage on arborvitaes does not necessarily mean the plants have pests. Water stress can cause similar symptoms. Be sure your plants get a deep watering once a week during the growing season. Too much water or soggy soils can cause browning foliage, as can lack of sufficient water.
Mites like dry, dusty places and the inside branches of arborvitae are common sites for them. The easiest control method is to simply hose your plants off during the hot, dry months of July and August. A strong spray of water, especially toward the inside branches and foliage will knock the pests off and discourage them from congregating there. Horticultural oil, insecticidal soap and neem oil are three "organic" sprays labeled for mite control on arborvitae. All three are available at most garden centers. These kill mites by smothering them as opposed to poisoning them.
Whether you're spraying soap or oil or just using the water-blast approach, the key is acting when the mites are present. A good test: stick a piece of white paper into the plants at several areas and tap the branches above. If little "dots" fall onto the paper and scurry around, those are most likely mites. One or two isn't too alarming, but if you see many of them, it's time to get busy.
Best wishes with your arborvitaes!
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