Question
My wild cherry had bag worms in the spring and now many twig branches are leafless, leaves are falling off and the many leaves remaining have small holes in them.  I fear gypsy moths but see no activity in the tree so am reluctant to spray without evidence that there is active damage being done now or is what I see a result of earlier attacks?

Answer
The "bagworms" sounds like eastern Tent Caterpillar. These insects make a web like tent over the leaves in the spring and defoliate the tree.  The eastern tent caterpillar is often mistaken for the gypsy moth. Though they are similar in appearance, they differ in habits.
The fully grown eastern tent caterpillar is about 2 inches long, black with a white stripe along the middle of the back and a row of pale blue oval spots on each side. It is sparsely covered with fine light brown hairs.

The gypsy moth caterpillar, when fully grown, is also about 2 inches long, but it has pairs of blue and red spots on its back.

Unlike the gypsy moth, the eastern tent caterpillar can be readily identified by the tent it constructs in the forks of tree branches.  Tent caterpillars spend the winter in egg masses that are in shiny brown bands around twigs.

The gregarious caterpillars hatch in the early spring about the time tree buds start to open, and soon they begin to spin their silken tents in the branch forks . The tent protects them from predators, such as birds, and from temperature extremes. Enlarging the tent as they grown, the caterpillars leave only to feed, usually at night.

The eastern tent caterpillar is found most often on apple and wild or ornamental cherry, and occasionally on pecan, Hawthorne, beech and willow. When abundant, caterpillars will eat all the leaves, weakening, though seldom killing a tree.

Winter is spent in the egg stage as one of many in a dark brown varnished collar or belt encircling the twigs. The young larvae hatch at or before bud growth in March or April and gather in a fork of the limbs to spin their large web nests. The larvae leave during the day to feed, but return to the nest at night, or during rainy weather. They are full grown (2 to 2 1/2 inches) in 4 to 6 weeks and often crawl to nearby buildings or protected places to spin their dirty white cocoon. The moths emerge in about three weeks and each female lays an egg collar (containing 150 to 350 eggs) around a twig. There is one generation per year. They overwinter in egg masses on small twigs.    

During the winter and prior to budbreak, susceptible hosts can be monitored for the presence of egg masses, which can be pruned out and destroyed. Also, newly forming webs in the spring can be pruned out or physically removed by hand, when the larvae are in the web. One should not use fire to burn out these webs; this can cause much greater injury to the host than this pest ever could. Bacillus thuringiensis (B.t) works well, especially on the younger larvae. The foliage around the tent should be treated since the Bt works when the caterpillar feeds on the foliage.

While heavy infestations can cause serious defoliation, eastern tent caterpillars rarely kill trees except those already weakened by disease or climate and environmental stresses.

Eastern tent caterpillar is more of a nuisance than detrimental to tree vigor. Feeding does not seriously damage healthy, mature trees ?the damage is primarily cosmetic, with trees appearing ragged or unsightly. Even if completely defoliated, most trees will leaf out again within two or three weeks, since caterpillar feeding generally ends during vigorous leafing activity. Small trees cannot tolerate as much defoliation without health consequences, yield on fruit tress will be reduced and trees already weakened by disease or environmental stresses may be killed. And the nests can be an eyesore in the landscape, particularly when exposed by excessive defoliation.