Bag worm infestation on small pine trees

I noticed that some of my small pine trees (4 ft.) started turning brown and upon closer inspection I noticed 100's of bag worms all over the trees. I spent quite a bit of time pulling off the bags. My questions are:

1. If I just throw them on the ground will they die or do I need to do more to make sure the bugs in the current bags don't come back?
2. What is the best insecticide or other method to keep these bugs from coming back?