Question
I planted a couple dozen arborvitae plants about four years ago that have recently been showing signs of distress.  In fact, two of them died recently and I noticed that the last one that died had many "cocoon-like" bags hanging on the branches.  When I cut them open, I found what appeared to be a rather large worm of some sort.  Could this be what has been killing my plants, and what can I do to prevent this from happening again.  I did remove and destroy all of the cocoons that I could find on all of my plants.  Than you for your advice and information.

Answer
This is an insect called a bagworm. They are really not killing the plants but eating the foliage, the trees should leaf out again.

The common bagworm, Thyridopteryx ephemeraeformis (Haworth), is an interesting caterpillar. The most commonly observed form of this pest is the spindle-shaped silk bag camouflaged with bits of foliage, bark and other debris (Fig. 1). Completed bags range from 1-1/2 to 2-1/2 inches long. The larva within the bag is brown or tan, mottled with black, and the bee-like adult males have clear wings and fur covered bodies. The females remain larva-like and do not emerge from the bag. The larva may stick its head and front legs out of the top of the bag to feed and move. When disturbed, the larva immediately pulls its head into the bag and holds the opening closed. Mature larvae may stay on their host plant or drag their bags some distance before firmly attaching the bag for transformation into the adult stage.


Bagworms have a single generation per year and overwinter as eggs inside the female bag. There may be 300-1000 eggs in a bag. The eggs delay hatching until late-May or early-June. As they hatch, the small blackish larvae crawl out the bottom of the bag and spin down on a strand of silk. These larvae on a string are often picked up by the wind and ballooned to nearby plants. When a suitable host plant is found the young larva immediately begins to form a new bag over its body. This bag is only about 1/8 inch long and is soon covered with sawdust-like fecal pellets.

As the larva feeds and grows it enlarges the bag and begins to incorporate bits and pieces of plant material. By mid-August, the larvae are mature and they often move to a sturdy branch or other structure where they attach the bag firmly with a strong band of silk. The larvae orient themselves with their heads down and pupate. The female pupa looks much like the larva while the male forms characteristic wing pads and other adult-like structures. Within four weeks the males emerge and actively fly about in search of females. Mature females call by releasing a sex attractant pheromone. After mating the female literally mummifies around the egg mass ,which remains in place until the next spring.

If the bags are few in number and easily reached they may be picked off the plant and squashed. This can be done easily in the late fall when deciduous foliage has been dropped or the bits of plant material on the bags turn brown and can be easily located on evergreens. Be sure to cut the attachment silk band so that the branch will not be girdled in the future.

Stomach insecticides are very useful for control of bagworms. Remember that the plant foliage is to be thoroughly covered because the larvae are protected from contact by being in the silk bag. Again, early sprays against young larvae are more effective than later applications. Products registered for bagworm control are: acephate (Orthene), bifenthrin (Talstar), carbaryl (Sevin),  diazinon, malathion, nicotine sulfate, pyrethrum, and  rotenone.

The bacterium, Bacillus thuringiensis (Bt), is effective against bagworms if it is used against young larvae. Applications should be made at the end of June after all the eggs have hatched and the larvae are through ballooning.