A heavy infestation of bagworms can defoliate a shrub and seriously damage a tree. The tiny caterpillars are hard to see and their 1 1/2- to 2-inch bags are camouflaged because they are made from parts of the plant. They may look like pine cones or other plant structures. Bagworm eggs overwinter in the bags, so removing the bags in winter is an effective method of control. Once they hatch, use insecticides to kill the caterpillars.
Winter or Early Spring
Pick the 1 1/2- to 2-inch spindle-shaped bags from the shrub.
Remove the bags and immerse them into a pail of soapy water to kill the eggs. Dropping them on the ground may allow some of the caterpillars to hatch and return to the host plant.
Search carefully for additional bags hanging from limbs and twigs. It is important to find them all. Each bag that remains on the evergreen allows hundreds of eggs to hatch into hungry caterpillars.
Middle to Late Spring
Apply insecticides early in the season when the bags are less than 1/2 inch long. Young bagworms are easier to kill than older ones, and killing them early prevents damage. Young bagworms are difficult to see, but if bags are present, you know you have caterpillars.
Find an insecticide labeled for use against bagworms. The University of Nebraska recommends low-risk insecticides containing Bacillus thuringiensis (Bt), neem oil or spinosad early in the season. For older bagworms, use an instecticide containing permethrin, acephate, bifenthrin, carbaryl, cyfluthrin, dimethoate, esfenvalerate, fluvalinate, malathion or tebufenozide.
Apply the insecticide according to the label instructions.