Bagworms have hatched in central Illinois, so it will be time to treat for these insects in central Illinois about June 10. This is 2 to 3 weeks earlier than normal. Now is the proper time to treat in southern Illinois, where the bagworms would have hatched a couple of weeks ago.
Bagworm eggs overwinter in the old female’s bag, which is close to 2 inches long and covered with dried foliage from the host. Newly hatched bagworms emerge from the old bag and climb upward in the tree. Then they spin out and hang at the bottom of silk threads that are typically 2 to 3 feet long. This swinging pendulum catches in the breeze, the top end breaks from the tree, and the young bagworm goes soaring through the air at the end of its buoyant silk strand. Caught in updrafts, they may float for very long distances—trips of many miles are probably common.
These newly hatched bagworms will continue to hatch and migrate for a couple of weeks. As such, they represent the only major method of dispersal for the species because the adult female is a wingless moth, unable to travel very far. Treatment of attacked trees and shrubs before they have finished migration will be only partially effective because new bagworms blow in daily. Typically, about 2 weeks after egg hatch, the bagworms quit migrating and settle down to feed on the foliage of arborvitae, spruce, pine, eastern red cedar, other junipers, and various deciduous trees and shrubs. Particularly on the needled evergreen hosts, defoliation can result in the death of branches and entire plants.
Spraying with Bacillus thuringiensis kurstaki (Dipel, Thuricide), cyfluthrin (Tempo), trichlorfon (Dylox), spinosad (Conserve), and other insecticides is effective, particularly while the caterpillars are small. As the caterpillars and their respective bags get longer than 1/2 inch, they become very hard to kill, particularly by insecticides other than those listed above.
Remember that you have a choice. If you spray at egg hatch, you will probably have to spray again to control larvae that migrate into the sprayed trees. If you wait for about 2 weeks after egg hatch, one application should be all that is needed, and little damage will occur during that time.