Question
Hello - we have a row of arborvitae along the northeast side of our yard.  They're rather shaded by a tall maple directly to their south.  They're also heavily infested with rust mites.  I knew the trees were browning and thinning, but I thought they were reaching the end of their lives at 24 years old.  Silly me!  The question is whether treatment is worth it - on my neighbor's side, the foliage is completely brown, and on my side they're partially so.  

I understand that we can eradicate these mites, but the question is whether enough foliage will be able to grow in to "green up" these trees, given their sad state of disrepair.  Thanks for your help.

Answer
Sounds like spider mites.
Spruce Spider Mite: Mites are not insects but are more closely related to spiders. Spruce spider mites (Oligonychus ununguis) are occasional pests of arborvitae. They are very small and not seen easily with the naked eye. They have piercing mouthparts that they use to suck plant sap. Their feeding results in speckling (formation of tiny yellow spots) on needles. Some needles may turn brown and drop off usually in the lower limbs.. With heavy infestations, fine webbing may be seen on the plant. Several seasons of heavy mite feeding may kill an arborvitae. Although most spider mites increase in numbers during hot, dry weather, spruce spider mites are cool-weather mites. Their populations peak during spring and fall, but drop dramatically during the heat of summer when predators feed on them.

To determine whether insecticide use is needed, it helps to know how many mites are present. Hold a white sheet of paper under a branch and strike the branch. The mites that are knocked off will be seen crawling around on the paper. If dozens of mites are seen per whack, serious damage can result. Continue to check population numbers at 7- to 10- day intervals. Pesticides labeled for homeowner use against spruce spider mite include insecticidal soaps and acephate + fenbutin oxide (Ortho Systemic Insect Killer or Ortho Orthenex Garden Insect & Disease Control Concentrate). Completely cover the foliage with the spray. As with any pesticide, read and follow all label directions and precautions before using.  

If it is spider mites if you treat the shrubs they should come back.