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University of Nebraska–Lincoln

UNL Extension in Lancaster County

Horticulture

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Bagworms

by Don Janssen, UNL Extension Educator

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The bagworm is a perennial insect pest of arborvitae, juniper, pine, spruce, and many other evergreen species. It also attacks certain deciduous trees such as black locust, honeylocust, and sycamore. The spread of the bagworm is slow since adult females are unable to fly. Their dispersal over wide areas occurs mainly through movement of infested nursery stock and ornamental plants, or by ballooning (wind dispersal) of small bagworm larvae during early June.

This insect is most easily recognized by the case or bag that the caterpillar forms and suspends from ornamental plants on which it feeds. The bag is made of silk and bits of host leaves and twigs. These materials are interwoven to disguise and add strength to the case. When the caterpillar is mature, the bag may be one to one and a half inches in total length.

Bagworms overwinter as eggs inside the female bag. Female bagworms lay 500-1000 eggs in each bag during the previous fall. Eggs will start hatching from late May through early June. Upon hatching, the young larvae crawl out of the bag and start to feed and construct silken shelters over their bodies. As the larvae grow over the eight to ten week feeding period, they continue to enlarge the exterior of their bags with pieces of foliage, bits of bark, or other plant parts.

Feeding and development usually continue until August. Mature larvae loop strands of silk around a twig and become firmly attached. During September and early October the males leave their cases and fly to bags containing females where mating takes place. Each mated female deposits a mass of eggs inside her bag. The eggs overwinter inside the bag until the following spring. There is only one generation a year.

Bagworm larvae injure plants when they feed on needles and leaves. Young caterpillars feed on the upper epidermis of host plants, sometimes leaving small holes in the foliage. Damage by mature larvae is especially destructive to evergreen plants. Trees such as sycamore, willow, and other deciduous trees, usually refoliate after heavy defoliation.

Unfortunately, bagworm infestations generally go undetected until damage is complete, and the large bags of these insects are very conspicuous. Early detection of an infestation requires careful examination of host plants for the presence of small bagworms attached to the leaves or needles.

Bagworms may be controlled on small shrubs and trees by handpicking or cutting the bags from infested plants during late fall, winter, or early spring, before egg hatch. Dispose of the bags so that this pest will not reenter your landscape.

When bagworms are too numerous to handpick, an insecticide application may be indicated. Several registered formulations of insecticides are labeled for bagworm control. These products should be applied from early to mid-June while the larvae are small. Otherwise, treatments will not be as effective against larger larvae. Pesticides are poisonous. Read and follow directions and safety precautions on labels.

(This resource was udpated May 2008 and appeared in the Lincoln Journal Star Newspaper Sunday edition. For information on reproducing this article or using any photographs or graphics, read the Terms of Use statement)

University of Nebraska-Lincoln Extension in Lancaster County is your on-line yard and garden educational resource. The information on this Web site is valid for residents of southeastern Nebraska. It may or may not apply in your area. If you live outside southeastern Nebraska, visit your local Extension office


Contact Information

University of Nebraska-Lincoln in Lancaster County
Web site: lancaster.unl.edu
444 Cherrycreek Road, Suite A, Lincoln, NE 68528
| 402-441-7180