Some bagworms are approaching 1 inch in length and causing considerable damage. Check to make sure that damage is not occurring, particularly on arborvitae, spruce, Eastern red cedar, and other junipers. Damage tends to be heaviest at the top of the tree, so use binoculars to aid inspection.
If you tend to use diazinon, chlorpyrifos (Dursban), malathion, carbaryl (Sevin), or other older insecticides, realize that bagworms over 3/4 inch long will not be controlled. Bacillus thuringiensis kurstaki (Dipel or Thuricide) should still be effective. Trichlorfon (Dylox), cyfluthrin (Tempo), and other synthetic pyrethroids should still be effective as well. In southern Illinois, the caterpillars may already be too big for Bt to be effective.
Other caterpillars commonly appear at this time of year and can be very noticeable. Yellownecked caterpillar, walnut caterpillar, white-marked tussock moth, and other caterpillars can occur in large numbers. They eat all the leaves off large branches or small trees. Realize that tree leaves provide most of their food for the tree during the first half of the growing season. Healthy, deciduous trees can lose some or all of their leaves from now through the end of the growing season with little impact on tree health.
Evergreens, particularly conifers, can be killed by defoliation so infestations there are more serious. Large caterpillar infestations on evergreens should be treated with Bt or another appropriate insecticide. Infestations of caterpillars on deciduous trees and shrubs may or may not be treated, depending on the health of the plants and their importance in the landscape. Plants on which the aesthetic damage of caterpillars would be very noticeable should be treated for that reason. Others, as long as they are healthy, probably do not require treatment.