Bt is a bacteria that causes the larvae to become sick, stop feeding, and later die. In St. Louis, the eggs hatch in late May to mid-June, or about the time the cigar tree, Catalpa speciosa, is in full bloom. Use chemical controls. Because bagworms form protective bags very early, contact insecticides, while useful, are less effective than stomach poisons. For best control, spray when insects are young.
Chemical controls become less effective as the season progresses.
Malathion, diazinon, and Orthene are effective.