If truly most of the needles are eaten, the tree might not be able to survive.
You should pull off and squish/throw away (in trash) all the bags you can reach.
Their life cycle determines the timing of control, so any spraying right now would not be helpful, but pulling those bags would be.
Here is our page on Bagworms for reference: http://extension.umd.edu/hgic/bagworms-trees-and-shrubs
Were they just planted? If it ever stops raining, offer supplemental water each week right up until the ground freezes for the winter.
This is a very mature tree, over 30 ft tall and part of a border screen around a 2 acre yard. Many tree now full of bags to high to reach and these trees get water from an underground sprinkler system. We sprayed in May/June and did not stop them!
Your question mentioned "new evergreens" so we weren't sure.
The non-toxic Bt can be applied early to small caterpillars and other controls can be used later in the season.
Without special equipment on trees that large though, it is unlikely that you can get good coverage by doing it yourself.
We'd suggest having a tree company do the job for you.