Question
I just learned that I have bagworms in 2 blue spruce trees on my property  in Eastern Pennsylvania and want to know how they spread and how I can prevent several other trees from becoming affected. The trees currently containing the parasite are on opposite sides of the property, about 50 yards away from each other but there are 3 other spruces in the general area.

Answer
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.
The bagworm occurs in the eastern United States from New England to Nebraska and south through Texas. The larvae seem to prefer arborvitae and red cedar but many other conifers and deciduous trees are attacked. These include: pine, spruce, cypress, juniper, willow, black locust, sycamore, apple, maple, elm, poplar, oak, and birch.

Bagworm females cannot fly and local populations can build rapidly when established on preferred hosts, especially arborvitae, cedar and juniper. Crowded larvae may eat the buds on these conifers causing branch dieback and open, dead areas. Excessive defoliation of these conifers may cause entire plant death during the following season. Moderate defoliation is unsightly. Since they are not as mobile as other moths they do not spread easily form tree to tree and normally you will have a population isolated on one or two tree side by side.

Cultural Control-Mechanical Hand Picking 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.

Use the Bacterial Spray Bt 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.

If you control the bagworms on these trees I do not think you will have problems with them on the other trees in the yard.  

Chemical Control-Insecticide Sprays 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), chlorpyrifos (Dursban), , diazinon, dimethoate (Cygon), , malathion, nicotine sulfate, pyrethrum, and rotenone..