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. After laying eggs, the female dies. The eggs overwinter in the bag with the dead female. There is one generation of bagworms a year in Illinois.