bagworms on juniper

Bagworms on Chinese juniper. Photo: Eric Rebek, Oklahoma State University, Bugwood.org

Updated: October 17, 2024

Bagworm, Thyridopteryx ephemeraeformis (Haworth), is a serious insect pest of many ornamental shrubs and trees in the eastern half of the United States. Conifers, especially arborvitae, cedar, juniper, and pine are the most frequently damaged host plants.

Deciduous trees such as sycamore, maple, locust, boxelder, and linden are also attacked but they are not seriously damaged. This pest is sometimes confused with Eastern tent caterpillar. 

Appearance and habits

  • Bagworms are actually the larval or caterpillar stages of moths.
  • After hatching, usually sometime in May in central Maryland, they immediately spin a small 1/8 inch long cocoon-like bag to which are attached pieces of leaves from the plants they feed upon. If you look closely, you will see them moving around as they feed on the plant.
  • Bagworms move about freely to feed, and they carry their bags with them. The bags gradually enlarge as they feed during the summer to house the growing caterpillar.
  • They spread from tree to tree by ballooning (they spin a fine web and use wind currents to infest nearby trees).

Life cycle

immature bagworms on juniper
Young bagworms on juniper foliage
  • In central Maryland, the eggs hatch sometime in May. Refer to the National Phenology Network's Bagworm Forecast for caterpillar emergence, which is based on Growing Degree Days.  
  • Bagworms complete their growth in August or early September. At this time, the 1-2 inch long bags are permanently attached to plant twigs by means of tough silken threads.
bagworm moth
Adult evergreen bagworm. Photo: Pennsylvania Department of Conservation and Natural Resources - Forestry , Bugwood.org
  • In late summer, they pupate inside the bags and then transform into moths, but only the males have wings.
closeup of bagworm caterpillar
Closeup of bagworm caterpillar. Photo: L.M. Vasvary
  • The males emerge from their bags in late summer and fly to the bags containing females. The males mate with the wingless females which remain in their bags.
  • Then each female lays 200 to 1,000 eggs in its bag and dies. The eggs remain in the bags until hatching occurs the following spring.

Damage

bagworm damage on arborvitae
Defoliation of arborvitae by bagworms. Photo: R Malloy, UME
  • When populations are high, bagworms are serious defoliators of plants. They cause permanent damage to evergreens. 
  • Shrubs and trees that become heavily infested, particularly conifers, may be killed.
  • Injury is not conspicuous early in the season because the caterpillars and their bags are small. The bags are not easily seen at this time unless large numbers are present. Bagworms often are not detected by the untrained observer until August after severe damage has been done.

Monitoring

Management

Mechanical control

  • On landscape shrubs and small trees, a simple method of control is to pick off the bags during the fall, winter, and spring.
  • Do not throw them on the ground near the trees but destroy them and throw them in the trash.
  • Remove the bags in any season but do it before the new generation hatches out in May or early June. 
  • When too many plants are involved, to make hand picking practical, sprays are in order.

Biological control

  • Bacillus thuringiensis, often called Bt, is a type of bacteria that only kills certain insects and does not affect humans or animals.
  • Bt must be applied by mid-July because it works well only on young bagworms.
  • Bt is commercially available under the following common brand names: Dipel, Thuricide, and others. Many of these brands are sold in local hardware stores and garden centers. 
  • Use according to label directions. Multiple sprays may be necessary. 

Chemical control

  • If chemical control is absolutely necessary, a registered insecticide should provide control if applied thoroughly to all infested plant foliage after July 15.
  • Check the label on the pesticide to be sure bagworm and the type of plant you wish to spray are listed.
  • CAREFULLY FOLLOW ALL LABEL DIRECTIONS WHEN MIXING AND APPLYING THE SPRAY.

Rev. 2020

Additional resource

Bagworm Damage | Ohio State University