|How would you get rid of bagworms on arbor vitae trees...|
A number of insecticides are effective in controlling bagworms. These include Sevin, Malathion and others. A more natural option would be a product containing B.t. of which there are numerous brand names. Anything you apply should be used while the bagworm is as young as possible. Their bags usually reach 1 1/2" to 2 1/2" in length. If you let them grow past about 1" they will be much more difficult to control. With the natural product B.t., this is even more the case, as older catapillars are not as easily controlled as young ones.
You may want to open a few bags to determine if the caterpillars are still alive. Repeat sprays 5 to 7 days later are often helpful.