Question
We have 25 emerald green arborvitae planted as a screen in 2001.  They were well until a few months ago when I noticed that a few on the end of the row were dying from the base of the shrub to about 5' up.  The remaining 3'-4' of the tree is full, green and healthy. All 25 look like they've been infested or diseased... but only from the base to half-way up the tree.  HELP PLEASE!

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.