Can an evergreen be saved/brought back to health after bagworms have killed/damaged 3/4 of it?
Damage Caused by Bagworms
It is iffy. But you have a chance. It is best, of course, to catch them very early in the process and your chances are better of evergreen survival. I don't use chemicals because birds eating them will be affected. I break open their webs so birds and wasps can get to them for lunch. But your case is extreme. Here is an organic solution. Not sure how it will work with such heavy infestation.
Webworms, tent caterpillars, loopers, green worms, sod webworms, army worms, leaf rollers and other caterpillars can be killed effectively with a spray of orange oil drench mix, spinosad products or any of the Bt (Bacillus thuringiensis) products. Add dd 1-2 ounces of molasses per gallon of spray. Even better is to prevent them from becoming a problem in the first place by releasing trichogramma wasps at leaf emergence. The first step is to tear the webs with a pole so the native wasps can eat them.