Bagworms are easy to control when they are young, and there is minimal damage to the plant. Look for bagworms in late June. Use the Fourth of July as a reminder date. The bagworms will be very small at this time and are easy to stop with almost any product. They may be as small as a grain of rice, so look closely for them. Removal by hand is the recommended bagworm treatment method. However, horticultural oils and soaps will work at this stage as well as BT, Bacillus thuringiensis. Sold under several names, such as Dipel. This is a natural control that only attacks the stomach lining of the caterpillar so it will stop feeding and die. Oils, soaps, and BT are the safest products to use since they are not harmful to people or pets.
As the bagworms grow larger, they develop a second stomach liner, and now BT is not effective. The use of traditional pesticides at this phase, unfortunately, is needed. However, if the caterpillar is out of the bag and feeding, one can use contact insecticidal sprays for bagworm treatment. They are out of the bag feeding in the early morning or early evening. During the day, they often hide from the sun inside the bag. When caterpillars are not feeding, using a systemic insecticidal spray (one that is absorbed into the plant leaves) is needed.
As the bagworm starts feeding on the leaves, it will ingest the plant leaf treated with the systemic insecticide and die. As the bagworm matures, it pupates and stays inside the bag until it emerges as a moth, lays eggs which hatch into the caterpillar, and the cycle starts over again. Once the bagworm pupates, insecticides have no effect, and the bag must be removed from the plant manually. A week after spraying, check the bags on the trees to make sure the worm has dried up and is indeed dead.
Read more about bagworms and other diseases that can affect evergreens by reading the Plant Wizard.