–by Dr. Raymond Cloyd
For those of you that have been waiting patiently by reading another book or counting your stockpile of toilet paper, or for some…impatiently, it is time to look for bagworms. Although the cool weather we have experienced this spring will slow development, and consequently larvae hatching from eggs, bagworm caterpillars will eventually be present throughout Kansas feeding on broadleaf and evergreen trees and shrubs. Therefore, be prepared to act against bagworms once observed on plants. Bagworms are primarily a pest of conifers; however, they feed on a wide-range of host plants including a number of broadleaf plants, such as; rose, honey locust, hackberry, and flowering plum. It is important to apply insecticides when bagworms are less than 1/4 inch long to maximize effectiveness of insecticide applications and subsequently reduce plant damage.
Several insecticides are labeled for use against bagworms including those with the following active ingredients: acephate, Bacillus thuringiensis subsp. kurstaki, cyfluthrin, lambda-cyhalothrin, trichlorfon, indoxacarb, chlorantraniliprole, and spinosad. Most of these active ingredients are commercially available and sold under various trade names or as generic products. However, several insecticides, however, may not be directly available to homeowners.
The key to managing bagworms with insecticides at this time of year is to apply insecticides early and frequently enough to kill the highly susceptible young caterpillars feeding on plant foliage (Figures 1 and 2). Applying insecticides weekly for four to five weeks when bagworms are first noticed will reduce problems with bagworms later in the year. The bacterium, Bacillus thuringiensis subsp. kurstaki, which sold under various trade names, is only active on young caterpillars and must be consumed or ingested to be
Fig 1. Young Bagworm Larva Or Caterpillar Feeding On Conifer (Auth–Raymond Cloyd, KSU)
Fig 2. Young Bagworm Larva Or Caterpillar Feeding On Plant Foliage (Auth–Raymond Cloyd, KSU)
effective. 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 (persistence). Spinosad is the active ingredient in several homeowner products, including: Borer, Bagworm, Tent Caterpillar, and Leafminer Spray; Captain Jack’s DeadBug Brew; and Monterey Garden Insect Spray. The insecticide works by contact and ingestion; however, activity is greatest when ingested by bagworms.
Cyfluthrin, lambda-cyhalothrin, trichlorfon, chlorantraniliprole, and indoxacarb can also be used against young caterpillars. Again, thorough coverage of all plant parts, especially the tops of trees and shrubs, where bagworms commonly start feeding, and frequent applications are essential in achieving sufficient suppression of bagworm populations. The reason multiple applications are needed is that bagworm larvae do not hatch from eggs simultaneously, but hatch over time depending on temperature. In addition, young bagworms can ‘blow in’ (called ‘ballooning’) from neighboring plants on silken threads. If left unchecked, bagworms can cause significant damage and ruin the aesthetic quality of plants. In addition, bagworms may kill plants, especially newly transplanted small evergreens, since evergreens do not usually 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 new 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.