Hello,

You may have arborvitae leafminers. However there are many other pests and diseases of arbs. The tiny insects you see may be mites, and there are parasitic ones that feed on leaves, and predatory ones that hunt other mites. Drought conditions, flooding, gnawing by rabbits or other wildlife at the base,  and root zone damage are other possibilities.

Please email your pictures to the MSU Plant Diagnostic lab. They will give an opinion based on what you can tell them- Such as: how long have these been planted?,  how much water the plant has received, any root zone disturbance nearby, any damage visible along the branch or at the base of the shrub on the bark?

The email is <personal data hidden>

Here is the website- https://www.canr.msu.edu/pestid/

If they need a branch sample they will contact you.

Here are references that explain more about the issues that cause dieback on arbs-

https://www.extension.purdue.edu/extmedia/id/id-477-w.pdf

https://ag.umass.edu/landscape/fact-sheets/arborvitae-needle-blight

https://extension.umd.edu/hgic/arborvitae-leafminer-trees-and-shrubs

Control options if it is leafminer damage:

https://content.ces.ncsu.edu/arborvitae-leafminer