The caterpillars feed on foliage near the old bag and construct a new bag with silken threads and bits of foliage taken from the host plant. Bags differ in appearance because of the plant material on which the bagworms are feeding. The bags made out of maple leaves look very different from those made out of arborvitae. The caterpillars add plant material to the bag as they feed. Accordingly, the bags grow as the caterpillars grow.
Young bags tend to point upwards whereas older bags hang downward.
Since the caterpillars are the mobile stage of bagworms, populations may be concentrated on one plant or adjacent plants. The wingless and almost legless female moths stay in the bags, while the black furry males , which have wings, leave the bags. Males fly in search of virgin female moths with whom they mate. Females remain in the bags and lay eggs inside the bags in autumn. A female may lay 500 or more eggs.