Answer: Thuja can be subject to spidermite damage so I'd check first for these pests before doing anything else. Spidermites leave webbing where the leaves meet the stems. The insects themselves are tiny (like a pepper flake) so identifying them without a magnifying glass can be difficult. Hold a piece of white paper under the inner branches and tap the plant sharply. Lots of debris will fall on the paper - look for tiny moving specks. Spidermites can be controlled with chemical sprays. To keep them from showing up year after year, occasionally hose your thuja off during the hot summer months. (Spidermites like hot, dry, dusty surroundings.)
Best wishes with your landscape!
Q&A Library Searching Tips