Bagworms have recently become very serious pests to junipers, arborvitae and even other deciduous trees and shrubs. The mild winters will not kill off as many eggs as usual and are blamed for this increase in population numbers. Bagworms live singly within a small spindle shaped bag that they carry with them as they feed. This bag is made from bits of foliage tied together with silken threads and offers protection from predators and parasites. Inside the bag is a worm or caterpillar (immature stage of a moth) that expands the bag up to two inches in length as it feeds and grows. When bagworms occur in large numbers, serious defoliation of the tree or shrub results.
This defoliation begins in the springtime but becomes very obvious by July or August. The best time to control these insects has passed (mid June), however, some control can still be achieved by using 'pyrethroid' insecticide applications or a new product called 'spinosad'. Both of these treatments have homeowner labels that mean that they can be purchased and used by the public without special licensing. Just follow the directions closely.
There are no systemic treatments that can be added to the soil or the trunk of the tree that offer consistent bagworm control. One of the most certain methods of control is still the age-old technique of physically picking the bags off of the trees and destroying them. This technique offers the homeowner several distinct advantages. First, it is absolutely effective. There is no question about them returning to life and continuing their feeding. Secondly, it reduces the expense and potential for harm that using pesticides might pose to people and the environment.
A concentration of dish soap in a small container is usually sufficient for this but some have devised other methods of disposal. I will not describe these methods here, but you can use your imagination. The bottom line is that when picking the bags off the tree or shrub, they must be disposed of. If dropped to the ground, they will simply climb right back on the plant again.