Spider mites once established and in high numbers can severely damage needled evergreens including Arborvitae. Mites are not closely related to our typical insects and are more difficult to manage. This one may be spruce spider mite, known to attack arborvitae as well. Non chemical treatments include weekly spraying the needles with a strong forceful stream dislodging the mites and syringing the foliage during hot dry spells when feeding damage is more visible. Stressed plants seem to be more attractive to the mites. Insecticidal soaps will also be effective, Be sure to get through coverage. Suggest a call to your local Extension Office for a more local and focus reply. Additionally Wisconsin may have products to use not allowed in Illinois.