I have a 75 foot run of emerald green arborvitae which I had planted 10 years ago. The past few years I've noticed small flies emerge when brushing up against the plant. I have looked for the insects on the plants but do not see anything. Until the past few years they were healthy and vibrant come the warmer weather. Now they seem to have at least some browning on them year round and seem to be getting worse. Any ideas?

The flies are there most likely because it is just a resting place. Your Arborvitae are beginning to show their age. It is natural to have browning older needles on the interior. If your plants have not been fertilized recently, feeding them in the spring of year when actively growing will give them a boost. Watering older plants during hot dry weather and not just during a drought will also aid in recovery. Evergreens planted closely together as is often done to create a barrier or hedge will increase the amount of browning as they grow together and lessen sunlight.