Bt for bagworms
Btk kills young bagworms feeding on arborvitae and juniper in early summer, while their bags are still only 1/8 to ¼ inch long. Maryland Extension says it must go on by mid-July, because it works well only on young bagworms. By August the caterpillars are too big for it. Pair it with hand-picking bags in winter, which removes the eggs before they ever hatch.
Before you order
A bacterium sprayed on leaves. Caterpillars that eat it stop feeding and die within a few days. It works only on the larvae of moths and butterflies, and has little or no effect on bees, lady beetles, lacewings or parasitic wasps. How it works, how to tell it has established and the supplier list are on the full Bacillus thuringiensis kurstaki profile.
Other controls for bagworms
No other reviewed control is listed for this pest yet.