Your article says May is the time to treat bagworms--across the country or where? We live in East TN. I understand Bayer tree and shrub will do the trick.
Bagworm insecticides such as Bacillus thuringiensis, spinosad, Sevin, permethrin or bifenthrin are listed as those commonly used.
Insecticide control is most effective at the time the eggs are hatching and the small larvae appear from their sacks. So it all really depends on a visual inspection and could happen anytime from April to June (the earlier end of this range for those in the south). If you wait to treat until the larvae are large, they won't be eradicated easily.