If your arborvitae is losing all of its internal needles and branches do not panic. Arborvitaes can be touchy plants at the best of times when planted in our heavy clay soils (as can all conifers especially leylands and cyptomerias). When they encounter very hot and dry conditions and are not adequately watered they begin to lose their internal foliage.Disease, insect or mites usually cause the outer foliage to brown or discolor.
Though this appears to be pest related it is actually just a normal response. If you want to avoid the problem just be sure to adequately water the plants consistently and monitor them closely during extremely hot and dry or just dry conditions to be sure they are receiving adequate moisture.
Unfortunately, this thinning is permanent though the plants will produce more external foliage which can mask the thinness.
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