As just hatched larva, they have a unique way of leaving home to look for new digs. They use the wind and a strand of silk they produce to float away and land on neighboring plants. As a very small larva, feeding damage is limited, as time goes along the bigger they get the more they eat every day until we see entire branches defoliated. When they have finished off one branch, the bagworm moves on to another and continues to eat. While all this feeding is going on the webbed bag decorated with bits of plant parts grows in size as the does the worm. They have been feeding for nearly three months before we really notice them. Until they mature that bag is carried with them with just their head and first set of legs poking out allowing them to move about and feed.
Once they have reached maturity as a larva, they will attach the bag permanently to a branch where they will then pupate into adults. The male finds the female. The female can lay anywhere from 300 to nearly 1000 eggs inside her silken bag.
Once they are at this stage, with the bag attached to the branch, treatments are not effective. Hand picking the bags from the host plant does a couple of things. It makes us feel better and by picking the bags off you are getting rid of 1000's of eggs that will hatch in 2017! While the bagworms and young and still growing and carrying that bag around, there are treatments that will work. One of the Bacillus thuringiensis strains, a naturally occurring compound will work. You will see this at your favorite retailer as Bacillus thuringiensis var. kurstaki or by the initials Btk. It is always better to treat as soon as you see feeding damage, so some weekly scouting is in order. There are other insecticides available too, just be sure bagworms are on the label and it is safe to use on host plant.
Bagworm populations will naturally rise and fall in the landscape so do not make any kind of treatments unless they are present.