Anyone who has ever seen bagworms (Thyridopteryx ephemeraeformis) on their arborvitae or junipers knows this is one strange critter. One week your arborvitae hedge is dense and beautiful the next it looks thin and has been decorated with what appear to be Christmas ornaments made out of dried leaves. The question is often asked--- where did they come from?
Inside each of these bags is a fast growing hungry caterpillar. The caterpillar is protected by the bag and moves, suspended from the branch, eating leaves or needles and occasionally adding to its home. The caterpillars and their bags start out very small but grow to about two inches as the insect reaches maturity in August.
Unlike most insects the female bag worm spends her entire life in the bag which she creates. Her eggs are laid in the bag; the female then exits the bag, dropping to the ground, where she dies. The eggs hatch the following season in May or early June. The young larva move out of the bag and start to weave their own bags. In some cases as the larvae spins its silk, the wind carries it off and deposits it on another plant— this is known as ballooning and is how these otherwise slow moving pests spread.
Look for and control bagworms in June. They are relatively easy to control when small. Minor infestations can be removed manually