After viewing some pictures a client brought in I was pretty certain of what I was seeing but really needed to make certain. The eleven year old hedge of Arborvitae was impressive but at a glance you could see the off color nature of the plants. Arborvitae are generally a nice green color but some of these were exhibiting a very off green to brown appearance. Upon a visit and a simple paper test (where I place a piece of white paper under the limb then tap the limb) I discovered a large spider mite population. The paper was covered with tiny black specks that begin to move around. It turns out that these particular ones are the Spruce Spider Mite and affect a ton of different evergreens in the landscape. For control methods see the UK publication Spider Mites on Landscape Plants.
Notice the Discoloration of the forward plant versus the on in the rear of the picture