Bagworms: they have arrived

Bagworms – Thyridopteryx ephemeraeformis – are emerging from eggs, and the young caterpillars are out and about feeding on plants. Eventually bagworms will be present throughout the rest of Kansas feeding on both broadleaf and evergreen trees and shrubs. Therefore, now is the time to initiate action against this insect pest.

Bagworms are primarily a pest of conifers but have expanded their host range to include a number of broadleaf plants, such as rose, honeylocust and flowering plum. Hand-picking small caterpillars (along with their accompanying bag) and placing them into a container of soapy water will kill them directly. This practice, if feasible, will quickly remove populations before they can cause substantial plant damage.

For those not really interested in enjoying the nice hot weather and hand-picking, a number of insecticides are labeled for use against bagworms, including those with the following active ingredients (trade name in parentheses): acephate (Orthene), Bacillus thuringiensis subsp. kurstaki (Dipel), cyfluthrin (Tempo), lambda-cyhalothrin (Scimitar), trichlorfon (Dylox), indoxacarb (Provaunt), chlorantraniliprole (Acelepryn), and spinosad (Conserve).

Most of these active ingredients are commercially available and sold under various trade names or as generic products. Several insecticides, however, may not be directly available to homeowners. The key to managing bagworms with insecticides is to apply early and frequently enough to kill the highly susceptible young caterpillars that are feeding aggressively on plant foliage. Older caterpillars that develop later in the season are typically more difficult to kill with insecticides.

Furthermore, females feed less as they prepare for reproduction, which reduces their susceptibility to spray applications and any residues. The bacterium Bacillus thuringiensis subsp. kurstaki is active on young caterpillars but the active ingredient must be consumed or ingested to be 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 reduces residual activity.

Spinosad is the active ingredient in a number of homeowner products, including Captain Jack’s DeadBug Brew and Monterey Garden Insect Spray. The insecticide works by contact and ingestion (stomach poison); however, activity is greatest when ingested. Products containing spinosad can be used against older or larger bagworm caterpillars later on in the season.

Acephate (Bonide Systemic Insect Control), cyfluthrin (Bayer Vegetable and Garden Insect Spray), gamma-cyhalothrin (Spectracide Triazicide), trichlorfon, chlorantraniliprole, and indoxacarb can be used against both the young and the older caterpillars. However, 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 eggs do not hatch simultaneously but hatch over a certain period of time depending on temperature, and 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 can actually kill plants, especially newly transplanted small evergreens, since evergreens do not usually produce another flush of growth.

Article by Raymond Cloyd, K-State Research and Extension Entomologist

Matt Young25 Posts

Matt Young is the Brown County Extension District director, as well as an agent in the area of agriculture and natural resources.

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