Bagworms overwinter as eggs inside the dead female's bag. Larvae hatch and emerge from the bags mid to late-May in Kansas. The young larvae begin feeding on their host plants right away. It's recommended to wait to spray for bagworms until mid-June to ensure most of the eggs have hatched. Young bagworms look just like the adult version only much smaller. If you see the empty bags as evidence of bagworms from the previous year on a plant, there are likely young bagworms on the plant this year as well.
Insecticides commonly used for controlling bagworms include:
Permethrin (38 Plus Turf, Termite & Ornamental Insect Spray; Eight Vegetable, Fruit & Flower Concentrate; Garden and Farm Insect Control)
Bifenthrin (Bug Blaster II, Bug-B-Gon Max Lawn and Garden Insect Killer)
Lambda-cyhalothrin (Spectracide Triazicide, Bonide Caterpillar Killer)
Spinosad (Conserve; Natural Guard Spinosad; and Captain Jack's Dead Bug Brew).
Spinosad is a very effective organic control for bagworms. Thorough coverage of the plant material is essential for good control of bagworms regardless of which product you choose. (Cynthia Domenghini)