–by Raymond Cloyd — Horticultural Entomology
We are getting close to the time when the 1/8 to 1/4 of an inch bags associated with the bagworm, Thyridopteryx ephemeraeformus, will be present on broadleaf and evergreen trees and shrubs. Therefore, be prepared to take action against bagworms once they are observed on plants. Although bagworm caterpillars primarily feed conifers, they also feed on a wide-range of host plants including many broadleaf plants, such as; elm, flowering plum, hackberry, honey locust, linden, maple, oak, rose, sycamore, and wild cherry. It is important to apply insecticides when bagworms are 1/4 of an inch long or less (Figure 1) to maximize effectiveness of insecticide applications and subsequently reduce plant damage.
Figure 1. Young Bagworm Larva or Caterpillar Feeding On Plant Foliage (Raymond Cloyd, KSU)
Many insecticides are labeled for use against bagworms, however, the insecticides that can be used to manage populations of bagworms early in the season are Bacillus thuringiensis subsp. kurstaki and spinosad. These active ingredients are commercially available and sold under various trade names. The bacterium, Bacillus thuringiensis subsp. kurstaki, is only active on young bagworm caterpillars and must be consumed or ingested to kill bagworm caterpillars. Therefore, thorough coverage of all plant parts and frequent applications are required. The insecticide is sensitive to ultra-violet light degradation and rainfall, which can reduce residual activity. Spinosad is the active ingredient in several homeowner products, including Captain Jack’s DeadBug Brew and Monterey Garden Insect Spray. The insecticide works through contact and ingestion, however, the insecticide is most effective when ingested by young bagworm caterpillars. The key to managing bagworms with these insecticides is to apply the insecticides early and frequently enough to kill the highly susceptible young caterpillars feeding on plant foliage. Applying insecticides weekly for four to five weeks when bagworms are first noticed will reduce problems with bagworms later in the year.
Bagworms commonly start feeding on the tops of trees and shrubs. Therefore, thorough coverage of all plant parts and frequent applications are important in managing bagworm populations. The reason multiple applications are required is that bagworm caterpillars do not emerge (eclose) from eggs simultaneously but emerge over time depending on temperature. In addition, young bagworms can be ‘blown in’ (called ‘ballooning’) from neighboring plants on silken threads. If left unchecked, bagworms can cause substantial plant damage, thus ruining the aesthetic quality of plants. Furthermore, bagworms can kill plants (especially newly transplanted small evergreens), because evergreens usually do not produce another flush of growth after being fed upon or defoliated by bagworms.
If you have any questions on how to manage bagworms in your garden or landscape contact your county horticultural agent, or university-based or state extension entomologist. You can also read the following extension publication on bagworms:
Cloyd, R. A. 2019. Bagworm: insect pest of trees and shrubs. Kansas State University
Agricultural Experiment Station and Cooperative Extension Service. Kansas State University; Manhattan, KS. MF3474. 4 pgs.
http://www.bookstore.ksre.ksu.edu/pubs/MF3474.pdf