Bagworms, is it too late to spray?
By mid-August, most bagworms have finished feeding and retreated into their bags. Insecticides will not penetrate the thick, leathery, silk-lined pouches.
To determine whether larvae have finished feeding, examine the bags. Those containing feeding larvae can be identified by the greenish, freshly chewed foliage glued to the bags. Bags are open at the head end, allowing larvae to poke out and continue feeding. They are attached to the foliage by several weak strands of silk and can be pulled off easily.
When larvae have finished feeding, they move to the twig or branch. They produce a heavy silken sheet that firmly attaches the bag to the host plant. These bags are more secure and difficult to remove.
Bagworms do not complete development simultaneously, so spraying for bagworms is not out of the question. Determining factors should be the condition and appearance of the host and the number of bagworms actively feeding.
If the tree has a full appearance and good color, and there are only a few bagworms actively feeding, do not spray.
Even if a tree has a thin, anemic appearance, check the condition of the bags. If most of the bags are worn and gray, they are the 2015 bags from which bagworms emerged. There may be only a few current year bagworms because, after consuming the most tender foliage, most larvae abandoned the tree and moved to adjacent trees to continue feeding on lush foliage. Spraying the tree is not recommended.
Even if most bags are this year’s, do not rush to spray a thin, anemic tree. Check to see if most bags are closed and attached with thick silken ties. If this is the case, do not spray the tree. Apply insecticide only if trees appear heavily damaged and bagworms are actively feeding. Check trees again next year to prevent further bagworm damage.