The secret to bagworm control is to spot the worms early in their development when they are caterpillars moving around the tree naked (no bag). Once the worms have taken refuge in their bags, they are harder to deal with.
Early In Season Control With Insecticide
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 if not impossible to kill with any spray.
So while you can still see caterpillars, spray tree or shrub needles thoroughly with a product containing eitherSpinosad or 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 Spinosad or 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.
Later Control Bagworms By Handpicking
It is easy to miss the first phase of the bagworm attack. You may not notice the bags until well into the summer. Then the simplest method of control is to handpick the bags containing the pests.
This a real challenge if the trees are large and the bags are far above your head. You may need to use pole pruners to clip off the ends of branches where bagworms are hanging. Destroy the pests by putting them in a plastic bag and discarding them in the trash. Placing the bag/insect between the cement and the bottom of your shoe is also a very effective bagworm eliminator.