For many areas in southern Iowa ("bagworm territory"), it is now too late to treat for bagworms. If the caterpillars are no longer visible and feeding, if the bags are no longer moving, then it is too late to treat. When the caterpillars have tied the bag to the twig, sealed it shut they are pupating inside and no further feeding will occur. The photos below from Madison County Extension and Jeromoy Baumbach show the difference of what bagworm bags look like when the caterpillars are active, and what they look like after the caterpillars are done feeding and growing for the summer. Note the sealed end and the silken strap that connected the bag to the twig in the lower photo.
When the caterpillars are no longer feeding, the tree will not experience additional damage beyond what has already occurred, and most importantly, spraying after that point is a complete and counterproductive waste of time and money. Even if caterpillars are still visible, spraying this late in the season may not be effective. Trees should be protected from bagworm defoliation by spraying in late-June to mid-July.
Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Horticulture and Home Pest News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on August 24, 2011. The information contained within may not be the most current and accurate depending on when it is accessed.