Key Features
- Chewed or skeletonized leaves
- Caterpillars in silken bags ("cones")
- "Cones" covered by cut leaf bits
Symptoms
During July and August, bagworms may defoliate conifers such as arborvitae and juniper as well as deciduous trees and shrubs. Bagworms are caterpillars that live inside spindle-shaped bags which they construct to protect themselves against birds and other enemies. These bags are made of silken threads and bits of foliage. Bags often go unnoticed when covered with freshly cut leaf bits that resemble the rest of the tree.
Biology
Early in June, when northern catalpa begins to bloom, the insects hatch from eggs that spent the winter in the old bags attached to tree branches. Caterpillars crawl out of the bag hanging from silken threads until they either reach a leaf or are blown to another tree by the wind. They start to spin their bags that they cover with leaves as soon as they begin feeding. The caterpillars poke their heads out of the bag to feed on leaves. Bags are enlarged as caterpillars grow. If disturbed, they retreat safely inside, and it is almost impossible to pull them out. Each female bag can produce up to 1,000 bagworms. Adult bagworms do not feed on foliage. Winged males emerge in August to mate with wingless females who keep their caterpillar form and never leave the bag. Mated adult females lay eggs in the bag in the late summer and early fall.
Management Recommendations
Bagworms tend to be a problem on trees that are isolated or in urban settings. When bags are found in the tree, simply pick the bagworms off and drown them in a bucket of soapy water. This method is most effective before eggs hatch out of the bags in June. When there are too many bags to pick, insecticides may be used to kill the bagworms. Pesticides are most effective before the bagworms are less than an inch and a half in length. To kill caterpillars without creating other pest problems (spider mites, scale insects) it is best to use biorational pesticides (spinosad, Bacillus thuringensis). Caterpillars may have to feed on treated leaves for 1-2 days to get a lethal dose of these materials. When pests are discovered late in season and extensive damage is present, it may be necessary to spray a rescue material to kill caterpillars quickly. Rescue treatments include Sevin(carbaryl) or a pyrethroid (eg. acetamiprid, bifenthrin, cyfluthrin, lambda-cyahalothrin,permethrin, resmethrin). Dinotefuran can be effective on small shrubs when applied to the soil in early May, three weeks before eggs hatch. To protect bees, do not apply foliar insecticides when bees are flying to flowers and avoid using soil insecticides like imidacloprid, and dinotefuran on plants prior to or during flower production.
Effective Pesticides
Active Ingredients include: Bacillus thuringiensis var. Kurstaki, Bifenthrin, Carbaryl, Chlorantraniliprole, Cyfluthrin, Dinotefuran, Horticultural oil (Paraffinic or superior oil) , Insecticidal soap (Potassium salt of fatty acid), Permethrin, Spinosad
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