Bagworm insects feed on a wide range of plants. Arborvitae and junipers are some the more commonly attacked evergreens. Feeding damage can be severe enough to kill the plants. The larvae are caterpillars that grow into moths. However, the adult female does not look like a moth. The female has no eyes, wings, legs, antennae or functional mouthparts. From the time the female larva builds her bag, she never leaves it. The male bagworms which are black with clear wings, leaves his bag and flies to the female, mates and dies several days later. The bag is a combination of silk webbing and parts of the plant. The bagworms over-winter as eggs. There can be between 500 and 1,000 eggs per "female" bag. The eggs hatch from late May to mid-June. Once the eggs hatch, the larvae feed on the upper part of the upper leaf surface. As the larvae grow they enlarge the bags and feed on the entire leaf leaving only the veins. Eventually, the larvae pupate in the bag.
An ichneumon wasp is a natural predator. However, the bagworm usually causes serious damage before the wasp can control very many bagworms. Bacillus thuringensis kurstaki is a bacterial disease on caterpillars including bagworms. Bacillus thuringensis kurstaki can be bought at many stores selling insecticides for the home landscape. When the commercial product is applied while the bagworm is still a larva, the bacterium can be very effective. Contact your local Extension Service for the proper timing in your area.