Treating is simple, although you may need a ladder.  Bagworms collect plant bits to make their traveling protection and cocoon.  When they pupate, they affix their bag to the stem of the shrub.  Although males have wings, females do not, so they stay in their bags where males visit to inseminate them.  Young larvae leave the bags to forage on their own.  Since neither females nor larvae can fly, spread is from branch to branch and tree to tree by walking.  Remove with clippers all the bags and throw them into the trash.  You will have interrupted the cycle.  NOW is the time to remove bags since larvae emerge in August through September.  Learn more at https://content.ces.ncsu.edu/bagworms.   Incidentally, because they are protected by their cases, pesticide treatment is not effective (except on early larvae in the spring). 

From NCSU: 

If the bagworms are first noticed in late summer, it is too late to do anything but physically remove the bags. By then they have already mated and females have already laid eggs inside their bags for next year's generation. It's best to use a very sharp knife or utility razor to cut the silk band that bagworms wrap around and around the twig just before they pupate inside. If you merely pull the bag to remove it, the silk band may slide along the twig and shear off the needles. If not removed, the silk bands sometimes girdle the twig, which causes it to eventually die and break off at that point. Bagworms removed from the tree should be kept in an open, dry paper bag to allow parasites to emerge from the removed bags to parasitize other bagworms in neighboring landscapes.

Late May or early June is a perfect time to spray for bagworms as then the caterpillars are tiny and very susceptible to pesticides. At that time, use a pyrethroid such as permethrin or bifenthrin because pyrethroids have a longer residual life than most other insecticides. When used as directed, pyrethroids are very toxic to insects but are not particularly hazardous to humans and pets (other than fish—avoid using pyrethroids around pools, ponds, and streams). These pesticides are readily available at most garden centers and big box stores.