Bagworms On the Loose

Damage - Neighborhood trees
Bagworm damage includes bronzing, defoliation, and sometimes tree death if not treated. (Photo: J. Green)

The evergreen bagworm, Thyridopteryx ephemeraeformis, is a major pest of coniferous or evergreen trees in Nebraska. These destructive caterpillars are called bagworms because they feed, grow, and live all, or most of their lives inside a bag. They remain active and mobile because they have full use of their thoracic legs while they feed on plant material from June to August.

Bagworms are a destructive pest of juniper, pine, spruce, arborvitae and other evergreen species. They will also feed on deciduous trees like shade, ornamental, and fruit trees, but because deciduous plants drop their leaves and grow new ones each year, the defoliation does not often kill the tree. Evergreen trees though, do not shed their needles and large populations of bagworms can kill the tree over consecutive years of feeding by mass populations.

Hundreds of tiny bagworm caterpillars in Nebraska emerge from their bag(s) in late May/early June. The emergence within in a geographic location is dependent on the temperature and accumulation of heat units, called growing degree days, so often by June, we have had enough heat units to stimulate bagworm caterpillars to emerge from bags. On most occasions, bagworms feed on the plant on which they hatched, or they are transported by wind to nearby trees or plants below. It is not unusual to see bagworms in home vegetable gardens or landscapes around schools, strip malls, or outdoor venues.

After they emerge, they feed and promptly find protection by covering themselves with a protective bag made from silk and their host plant material. They feed inside this bag, protected from predators, the elements, and contact insecticides. When they are disturbed they can retract and cinch up the bag, which is already highly camouflaged because it is covered with fresh plant material from which the caterpillar is feeding. They feed and grow, and as they do, the bags grow too.

Mature caterpillars stop eating in August and attach the bag a branch or twig with a strong strand of silk and pupate inside. Male bagworms pupate and emerge as fuzzy, brown moths in the fall. Females bagworms transform into a wingless moth, but do not leave the bag. Males moths locate the female bags by a pheromone that she produces, mates with her, and dies shortly thereafter. The female bagworm who spends her entire life inside the bag, lays hundreds of eggs (some sources say up to 1000 eggs) inside the bag before dying. The eggs overwinter inside her pupal case and layers and layers of think silk and plant material. There is one generation per year.

Dealing with bagworms can be extremely frustrating because:

  1. Bagworms remain incognito until major damage is detected.
  2. Newly emerged bagworms can disperse with the wind from nearby trees.
  3. Bagworms are difficult to control in large trees (like windbreaks) because they are cannot be reached to physically remove or treat with insecticides.
  4. Bagworms remain protected in the bag and the window for effective insecticide treatment is small (after caterpillars emerge but before bags are 1/2-inch long).
  5. Bagworms are not picky about their host plant and will feed on all types of plants.
  6. In 2019, bagworms were seen hitching rides on cars, dangling from mature deciduous trees, and attached to exterior walls and eaves of buildings.

Management for bagworms include:

  • Cutting or handpicking the bags off and destroying them before caterpillars emerge late May.
  • Destroy bagworms by dropping them in a bucket of soapy water or seal them in an airtight bag that they cannot escape.
  • Bacillius thuringiensis (kurstaki) and spinosad are biorational insecticides that can be applied to foliage to kill young caterpillars as they feed. Biorational insecticides kill caterpillars without causing harm to natural enemies.
      • Must have complete coverage of foliage
      • Must be consumed to be effective
      • Works best in June when bagworms are small
  • After the window/after the month of June when bagworms bigger, conventional insecticides need to be used and these may include malathion, acephate, carbaryl, bifenthrin, permethrin etc.
  • There is also a preventative, systemic soil treatment, which must be applied to the base of the tree weeks before caterpillar hatching. The active ingredient is dinotefuran and may need to be treated by a certified arborist/applicator.

Now that bagworms have hatched in our area, we want to keep a watchful eye on our trees and shrubs.

Bagworm Resources:

Backyard Farmer Video Segment (3:40): https://youtu.be/LCohIuo8H1Y

Purdue University Extension: https://extension.entm.purdue.edu/publications/E-27/E-27.pdf

UNL Extension: https://lancaster.unl.edu/hort/bagworm.pdf

Jody Green
Extension Urban Entomologist at Nebraska Extension
Jody Green is the extension urban entomologist at Nebraska Extension in Douglas-Sarpy Counties. She specializes in structural, household, and health-related insect pests.
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