Bagworms are typically harmless...until they infest any conifer. They will literally kill/severely damage any cedar, spruce or evergreen they get on, as these shrubs and trees do not regenerate. Bagworms prefer a cedar to a deciduous tree, too. To save such, the shrubs and trees must be sprayed at the first sight of a hatch. There will be thousands of them, too. They spread by ballooning on silk. Female moths NEVER leave the cocoon. They are wingless and entombed. They are fertilized in the cocoon, their hatchlings leave and spread, just like baby orb weaver spiderlings from a common egg mass.
On the upside, bagworms are great biological control for infestations of the often noxious and highly invasive eastern red cedar. One can colonize them by collecting/attaching fresh fall cocoons to one in every 10 red cedars and just let nature take its course.