It appears that you may have a few problems going on. One possibility is a fungal disease. For this: Avoid wounding twigs. Prune out and dispose of infected shoots during dry conditions. Keep foliage dry by using drip irrigation and providing good air circulation. Avoid fertilization.

It also appears you may have a spider mite problem. For this: Dormant oils can be used to smother the eggs in the fall or early spring.  Many growers target the egg hatch to apply ovicide/larvicides for long-lasting effect. These mites prefer cool temperatures and can build up rapidly at optimal temperatures (60-70 F). Damage often shows up in summer after heavy spring feeding.

Also check out the possibility of a leaf miner problem. For this dormant oil can be used as sated above.

The following links have useful information:

http://pnwpest.org/pdf/reb141.pdf

https://extension.psu.edu/arborvitae-diseases

https://extension.umaine.edu/ipm/ipddl/plant-disease-images/arborvitae-arborvitae-leaf-miner-signs-symptoms/

http://www.clemson.edu/extension/hgic/pests/plant_pests/trees/hgic2004.html

Hope this helps!

 

Thank you so much for your advice and the informative links you provided. I have had to remove the diseased tree from my yard. While removing dead branches from tree I discovered tiny holes ( 1mm) in the bark. Hundreds at least. When I peeled the bark I saw larva and even adult beetles, the adults being about 2mm long. this pest had girdled the tree with their tunnels under the bark.This beetle has killed one arborvitae and is working on the  other eight in our yard. The trees are about fifteen feet tall and twelve years old. I just removed the dead one and fertilized the remaining trees with spike type fertilizers. I will try to remove any deadwood from others. Can you advise on further treatment ?

Bark beetles are very small insects, usually less than 8mm. They are brown to black in color, and cylindrical in shape. The beetles generally attack stressed or weakened trees. The immature beetles construct galleries as they feed on the cambium, under the bark. Healthy trees can usually withstand bark beetle attacks by literally trapping the beetles in a sticky flow of pitch.

Management: The best control for bark beetles is prevention. Healthy trees are able to withstand most bark beetle attacks. Insecticidal sprays will not kill the beetles or larvae inside the tree and sprays will not reverse the damage that has already occurred. Trees should receive adequate water in times of summer drought. Dead or dying trees should be removed to eliminate breeding sites for additional beetle generations.  Many trees succumb to bark beetle attack during drought years. Trees that are infested but not yet showing symptoms will probably die the following year.

You should remove all your infected trees. Yikes!!