I received a call the other day about what to do with a blue spruce covered in bagworms. The tree is apparently covered (and large), so hand-picking was not an option. The caller said the pest was rather large, too, so I had to recommend a systemic insecticide as the treatment.
I prefer that we catch the bagworms earlier in the season so that we can take a biological approach, but this is not always the case.
Like most caterpillars, bagworms are voracious eaters as they store up calories for growth, metamorphosis and reproduction. Now is the time they do the majority of damage to evergreen trees and shrubs.