There are several bagworm species, but the most common one is the evergreen bagworm (Thyridopteryx ephemeraeformis). The adults are moths and have no distinguishing features. You normally spot them during their larval (caterpillar) stage by the portable bags they build out of bits of leaves and twigs held together by very tough webbing. They look like round spears and are usually brown in color. They start out small at the beginning of summer, but grow to be over 1-2 inches long as the worm expands its home. They crawl around on trees, eating the foliage, with only their head and legs sticking out of the bag. When at rest they retreat completely inside. Large populations of the little buggers can do serious damage ,through stripping the tree of leaves. Evergreens are most often attacked, but I have seen them work on maples as well.
Late in the summer the larvae pupate inside the bags and become adults. The females are wingless and normally don’t leave their home until eggs are laid in the bag.
The insects can be controlled by a number of insecticides, including Sevin, Orthene, and Malathion. Normally two applications are needed for good control, and be sure to follow label directions. If you want to go the organic route you can try Dipel insecticide, or pick off the bags and drop them in kerosene or give them a good stomping.
For insect or disease problems on trees and shrubs, contact your local state Forestry office or County Extension Agent.