Bagworms are whitish caterpillars that construct individual silk cases around themselves. They cover these silk cases with bits of leaves from the tree or shrub they are feeding on. These bits of foliage turn brown within a couple of days, resulting in brown bags moving across the trees and eating leaves.

Throughout its entire life as a caterpillar, this insect places new foliage at the top of the bag. Thus, an actively feeding caterpillar has green foliage at the top of its bag. If the caterpillar pupates, it ties off the top of the bag to a branch, and any green foliage quickly turns brown. If the caterpillar dies, the foliage on the bag will be completely brown. Bagworms that are alive and actively feeding can be controlled with trichlorfon (Proxol, Dylox), cyfluthrin (Tempo), and other synthetic pyrethroids. Bacillus thuringiensis var. kurstaki (Dipel, Thuricide) is more spotty in its control of large larvae but is the only option for do-it-yourself homeowners.

Bagworms start to feed at the top of the tree and work their way down. A pair of binoculars is useful for detecting feeding bagworms on large trees. Bagworms most commonly attack eastern red cedar and other junipers, as well as spruces, arborvitae, Douglas-fir, honey locust, pin oak, red oak, and tallhedge. Stripped branches of coniferous evergreens will probably die.

Bagworms will pupate later this summer. Male bagworms emerge as black, one-inch-long moths with clear wings. They mate in early fall with the adult females that stay in the bag. Adult female bagworms are brownish and larval in appearance. Mated females fill their bodies with up to 1,000 eggs in the fall before they die. These eggs hatch in June of the following year.

Old bags that housed males will have dark brown pupal cases (about 1/2 inch long) sticking out of the end. The other bags are likely female bags that can be picked off trees from late fall through spring to reduce the number of caterpillars that are present next year. Do not just toss the picked bags to the ground under the tree because young bagworm larvae crawl long distances and will probably crawl up onto the tree. Picking off all of the old bags will not totally eliminate bagworms from the tree because young bagworm larvae are blown on strands of silk from tree to tree for a couple weeks after they hatch in June.