The name sounds disgusting, doesn't it? Actually, they don't look that menacing when you see them on the plant. In fact, they can be easily mistaken for a cone or part of a plant. Don't be misled though - these devious critters are highly destructive and can kill a plant before you realize what has happened. There are different types of bagworms but the evergreen bagworm is the one addressed here. It is prevalent in the eastern U.S. and attacks evergreens such as arborvitae, red cedar, junipers, etc. Even decidious trees are vulnerable, however, damage is minimal to them because they shed their leaves.
This is what they can do -
Bagworms are actually the larvae of a moth. They are 1-2 inches long and create a cocoon made up of silk and bits of dead foliage and twigs. During the summer months, the larvae consumes plant foliage and retreats into the cocoon when disturbed. The larvae carries the cocoon (or bag) around and it larger and larger as they eat more and more. By late August, feeding stops and a pupa forms inside the bag. A moth emerges from the pupa. The female moths stays inside the bag. The male flies to bags containing the females and they mate. The female then lays 300-1000 eggs in the bag and then she promptly dies. The eggs hatch the following May and early June and the whole vicious cycle starts over again.
You can control them by spraying an insecticide but this has to be done in May and early June to be effective. The larvae are so small, most gardeners don't see them until the bags start to form. Feeding slows in August so spraying doesn't do any good after that.
Hand picking them is another alternative. Of course, if it is a large tree, that option might be out of the question.
A few years ago, I noticed bagworms on our Colorado Blue Spruce. I picked them off and never saw them again. This year I've noticed them on the junipers out next to the street. This morning I spent an hour picking them off and tossing them into the road where I envision car tires squishing the disgusting buggers to kingdom come.