Control with Bt (Bacillus thuringiensis)
In their worm or caterpillar stage bagworms are vulnerable to residual insecticides only while they are still moving around dragging their bags and eating leaves or needles. Contact insecticides are very difficult to use against this pest. It gets worse, as soon as they close the entrance to the bag, making it extremely difficult to kill with any spray. So while you can still see caterpillars, spray tree or shrub needles thoroughly with a product containing Bacillus thuriengensis (BT) when the worms are actively feeding. This will likely be sometime between May 1 to June 1, depending where you live. The bagworms eat the Bt, which is a bacteria that affects their digestion, when they chew on the leaves of the tree. They stop eating soon and die within a few days. Spray the entire area where the caterpillars are feeding as directed on the product label. In situations where the tree or shrub is very tall and difficult to reach, hire a professional arborist in the early spring to spray them properly and safely.