A common and easy to recognize pest of many evergreen and other trees and plants is the bagworm. As its name implies the worm or larvae lives in a bag-like structure. The bags are made of silk and incorporate bits of leaves and twigs from the plants they live on. Some people describe the 1 to 2 inch bags as looking like Christmas tree ornaments dangling from the branches. The tough, silken bag becomes the overwintering home for hundreds of eggs which will hatch the following spring. The bagworm eats foliage. Plants can be partially defoliated to completely defoliated when there is a high population.
The least expensive way to control bagworms is to pick them off the plant and destroy them. If there is a large population this may not be practical. There are several insecticides labeled for bagworm control. A biological control is Bt, which stands for Bacillus thuringiensis. Bt is a bacteria that causes the worms to get sick, stop feeding and die. For best control the bagworms need to be treated early in their life. They generally start hatching in mid to late May and hatch through mid June. For more information on bagworm control contact your county Extension office, Master Gardener hotline or local nursery.
Reference: MU Guide G 7250 The Bagworm in Missouri