Bagworms attack a wide range of evergreen and deciduous trees. They also attack shrubs such as arborvitae, juniper, spruce, fir, pine, maple, box elder, and black locust. The young caterpillars (larvae) emerge from overwintering eggs in June and start feeding. They create a bag or pouch made of interwoven dead foliage from host plants, small twigs, and silk, and these bags cover the caterpillars for the rest of their lives. Young caterpillars are difficult to see because they blend in with plant foliage. They can be blown onto other trees by wind. Large populations of bagworms can completely defoliate plants. As the caterpillars increase in size, the bags dangle from trees like Christmas ornaments. In August, males pupate within the bags and then emerge as winged adults. It takes approximately 7 to 10 days for a bagworm to change from a pupa to an adult, depending on the temperature. The males then fly off to mate with females in bags. After mating, females remain inside bags and produce eggs. A female bagworm can lay between 500 to 1,000 eggs; then the female dies.
Management of bagworms includes physical removal and/or the use of insecticides. If small populations (less than 30 bags) are present, the bags, which hold the overwintering eggs, can be physically removed before egg hatch. Bags should be placed in a plastic container and disposed of. Insecticides should be applied about two weeks after hatching starts to allow all the bagworms to hatch and blow around. Because it appears to be an early spring, insecticides can be applied now in southern Illinois and in mid-June in central Illinois. Bagworms are not usually found in high numbers north of I-80.
Insecticides that are effective in controlling bagworms include Bacillus thuringiensis kurstaki (Dipel) and spinosad (Conserve). The bacterium Bacillus thuringiensis is effective on young caterpillars but the material must be ingested so thorough coverage of all plant parts is essential. Spinosad works by contact and ingestion. My research with Dr. Clifford Sadof at Purdue University demonstrated that spinosad is very effective in controlling bagworms. In addition to these two insecticides, research has shown that certain species of beneficial nematodes attack bagworms. When the nematodes are sprayed onto the bags, they infect the bagworms inside. The bags provide a humid environment that is conducive to nematode activity.