The bagworm is actually the larvae form of a small moth. This larvae looks like a small green caterpillar that eats tree leaves. Then the larvae makes a cocoon that resembles a hanging bag as it turns into a pupae inside the bag until it emerges as a moth. Then the short lived moths lay eggs on the host tree to start the cycle again. The easiest time to control this pest is at the green caterpillar stage in the spring. Just pick and squish any worms you see and encourage the birds in your area to finish the job. If you do see bags hanging on the trees pluck them off and destroy them. A few bagworms on a tree will not damage the tree much, so you could be okay. If the problem returns next spring spray with BT or Bacillus thuringiensis which is a bacteria that is used as a biological pesticide. You can also use a dormant oil spray on the bags in the winter once the leaves have fallen from the tree. One more idea is to plant asters near the infected trees. The asters attract a certain fly that will feed on the bagworms.  Good luck and Keep Growing, Marianne Binetti