Answer: What you describe sound like bagworms and they are quite common in arborvitaes and other needled evergreen shrubs.
Bagworms are 1/8 to almost 2 inches long depending on age. Each bagworm is covered by a bag made of white silk with bits of the host plant spun onto the outer surface.
They hatch in May and June. The newly hatched larvae spin down on silken threads and are blown about by the early spring breezes. Most of the larvae land on the original host plant but some small worms may be "ballooned" for some distance on the silk thread. Upon reaching a suitable host, the worm begins to spin its bag and as it grows, it incorporates some of the host plant foliage into the bag for camouflage. As the bagworms grows, it enlarges the bag and adds fresh plant material to the outside. In August the worms mature and molt into the pupal stage. The bag is firmly attached by a sturdy silk band which the bagworms usually wrap around a twig.
While they are young, bagworms can be fairly easy to control. Handpicking can be tedious, but it is also effective to the extent you can reach. If you can?t reach them all and if you don?t object to insecticides (there are organic options), then now is a good time to spray. The organic Bacillus thuringiensis (B.t. ? marketed as Biobit, Dipel, Foray, and other brands) is effective but only on young caterpillars that are feeding. They have to eat it. As they get bigger, they eat less; and it takes more to kill them. So the sooner you treat, the more likely you are to be effective. Once they get a little larger, they are more difficult to kill. Then you?ll have to go to things like Sevin or malathion.
Best wishes with your landscape!
Q&A Library Searching Tips