Answer: Because it is so late in the growing season, I would simply hose the plants off to remove most of the live spider mites and, hopefully, their eggs. Next spring, before the remaining eggs hatch, you can use insecticidal soap to smother them. Your arborvitaes may be sensitive to insecticidal soap so I'd recommend spraying according to label directions and then hosing the residue off the plants after about 10 minutes. Insecticidal soap will kill the pests on contact but because it has an oil base, it can cause problems with the foliage if repeated sprays are needed to control the spider mites and the oily residue has a chance to build up. I think the combination of regularly hosing off your arborvitaes all summer long, and a couple of spring sprays with insecticidal soap will control the pest problem.
Best wishes with your arborvitaes!
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