Bagworms are caterpillars that make distinctive spindle-shaped bags on a variety of trees and shrubs. Twenty species of bagworms exist, and they attack both deciduous trees and evergreens, but are especially damaging to juniper, arborvitae, spruce, pine and cedar.
Although numbers have declined recently, residents are encouraged to be vigilant. Bagworms are more easily identified in the winter and spring (from a distance they can be mistaken for cones). They can be easily removed and destroyed -- simply pick the bagworms off the plant and squash them. Be sure to cut the attachment silk band so that the branch will not be girdled in the future.
Bagworms spread slowly because the female is unable to fly, but they can be blown by the wind or crawl to other host plants. Landscapers may unintentionally transport bagworms in the process of pruning and moving infested plant material.
Treating Severe Infestations
Severe infestations may require treatment. Spray applications of a bacterial insecticide such as Bt (Bacillus thuringiensis) are effective in controlling bagworms. Applications need to be administered in June, when the spray-timing indicator plants Northern Catalpa (Catalpa speciosa) and Japanese Tree Lilac (Syringa reticulata) are in full bloom. Applications are repeated at 3 - 10 day intervals, dependent on weather. At least 2 applications are recommended.
Bagworms are often compared to caterpillar larvae, as the worm hides inside a self-constructed bag likened to a cocoon (hence the name bagworm). Bagworms create these cases out of silk and bits of twigs or leaves, interwoven to disguise and strengthen the case. When the larva is small, it feeds on the upper side of the leaf, resulting in a brown spot on the leaf. The larva moves to the lower surface as it grows and eats all of the leaf except for the larger veins. The change from pupa to adult requires 7 - 10 days, depending on weather.
Adult females never leave the bag they constructed as a larva, but adult males emerge as moths. The male moth flies to the female, mates, and dies in a few days. The female produces 500 to 1,000 eggs that overwinter inside her bag. The eggs hatch and larvae emerge in late May to early June.
Additional Bagworm Information
For more information, contact Jim Johnson, Village Forester, at 630-671-5804. Color photos can be found at the Forestry Images' website.