The time is approaching when the 1/8 to 1/4 of an inch bag of the bagworm, Thyridopteryx ephemeraeformus, caterpillar (larva) will be present on broadleaf and evergreen trees and shrubs. Hence, you should be ready to “take action” against bagworms when they are observed on plants. Bagworm caterpillars (larvae) feed on conifers, but they also feed on a wide range of broadleaf plants, including: elm, flowering plum, hackberry, honey locust, linden, maple, oak, rose, sycamore, and wild cherry. Apply insecticides when bagworms are 1/4 of an inch long or less (Figure 1) to maximize effectiveness of insecticide applications and reduce plant damage.
There are several insecticides labeled for use against bagworms; however, the insecticides commonly recommended to manage bagworm populations 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 (Figure 2), kills young bagworm caterpillars after they have consumed or ingested the bacterium. The insecticide is sensitive to ultraviolet light degradation and rainfall, which reduces residual activity (persistence). Consequently, repeat applications may be required. Spinosad is the active ingredient in several homeowner products, including Captain Jack’s DeadBug Brew (Figure 3) and Monterey Garden Insect Spray. Spinosad works through contact and ingestion but is most effective when ingested by young bagworm caterpillars. Managing bagworms with these insecticides involves applying the insecticides early in the growing season and making frequent applications to kill the young caterpillars feeding on plant foliage. Apply the insecticides weekly for three to four weeks when bagworms are first noticed to avoid problems later in the growing season.
Bagworms typically 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. Multiple applications are usually required because bagworm caterpillars do not emerge (eclose) from eggs simultaneously. Instead, they emerge over time depending on temperature. In addition, young bagworms may 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 (Figure 4). In addition, 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. For small plantings with minimal bagworm infestations, you can hand remove the 1/8 to 1/4 of an inch bags containing the caterpillars and squish them between your fingers.
For more information on bagworms, you can access and read the following extension publication:
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 pages.
http://www.bookstore.ksre.ksu.edu/pubs/MF3474.pdf
By Raymond A. Cloyd, Professor and Extension Specialist in Horticultural Entomology/Plant Protection