Bagworms will have hatched in southern Illinois. They should hatch by mid-June in central Illinois. When newly hatched bagworms emerge from their mother's bag, they climb to the top of shrubs, trees, and any other erect object. They spin out two to three feet of silk which catches in the wind and blows them to new locations. They repeat this process, called ballooning, for about two weeks before settling down to serious feeding.
There is little to gain from spraying for bagworms during ballooning because most insecticide residues under the high sunlight and temperatures at this time of year only last for a few days. This allows later arriving ballooning caterpillars to feed and survive due to insecticide breakdown.
Treatment will be most effective in mid-June in southern Illinois, during the first week of July in central Illinois and a week later in northern Illinois. Bacillus thuringiensis kurstaki (Dipel, Thuricide), spinosad (Conserve), cyfluthrin (Tempo), permethrin (Astro), and other pyrethroids are effective even on older larvae. They are more effective on younger larvae, so treatment soon after they stop ballooning is recommended. (Phil Nixon)