Bag worms or tent caterpillars? Just wondering.

As long as your plants are generally healthy, they can usually tolerate a defoliation. Repeated events are what cause a problem, or a defoliation on top of drought conditions, etc.

At this time of year trees will be munched on by "Bag Worms" and "Fall Webworms". Both, early in their life cycle are very susceptible to Bacillus thuringiensis - Kurstaki, but it is not effective after they reach about 3 weeks of age.
Whether your tree(s) will be adversly affected depends on how stressed they are by other environmental factors, ie. drought. Defoliation of deciduous trees at this time of year is not as harmful as some think since the trees have stored the energy necessary for future growth and the trees leaves are getting ready to drop anyway.
Those trees will grow leaves again next year, and to help control these wee buggers in early July you can apply a band of a sticky substance around the tree trunk to capture the female moth that will lay the eggs that cecome the Fall Webworms.

bigdope, have you decided if you have bag worms OR tent caterpillars? It makes a big difference for your control strategies.