Bagworms (Thyridopteryx ephemeraeformis) are moth larvae that feed on more than 125 species of trees and shrubs, often defoliating and even killing the plants. This menace is hard to spot because it only reaches about 1/8-inch long, but you can easily recognize the caterpillars' presence by the silken bags hanging from branches. Although mature bagworms are difficult to control, you can reduce pest populations by using biological, mechanical and chemical means while the worms are still small.
Encourage Natural Enemies
Bagworms have numerous natural enemies that can help control light infestations. Insect predators include tachnid flies and parasitoid wasps. Attract those beneficial insects to your yard by placing flowering members of the aster family (Asteraceae) around infested trees and shrubs. Shasta daisies (Leucanthemum x superbum) and Frikart's asters (Aster x frikartii), both of which grow in U.S. Department of Agriculture plant hardiness zones 5 through 9, are just two colorful options. Several bird species, including sparrows, sapsuckers, finches and woodpeckers, also feed on bagworm larvae. Entice birds to hang around your landscape by supplying them with birdbaths, bird feeders and birdhouses.
Handpick Bagworm Bags
Control bagworm infestations on shrubs and small trees by removing bags in the autumn or winter when the foliage has dropped and you can easily spot the protective cases. Finish handpicking by early spring so you're done before the eggs start hatching. Destroy the bags by dropping them into a bucket of soapy water or crushing them underfoot. Don't just drop the bags on the ground or the worms will simply return to their host plants. Cut the silk band that attaches the bag to the limb so it won't girdle the branch as it grows. If you choose to handpick, make sure you remove every bag. Each bagworm bag contains 500 to 1,000 eggs, so missing just a few can lead to a severe infestation. Combining handpicking with insecticide applications often offers the best pest control results.
Spray With Bt
Bacillus thuringiensis is one reduced-risk pesticide option that has proven to be very effective against young bagworm larvae. Bt is a soil-borne bacteria that kills caterpillars without harming beneficial insects, humans or pets. Carefully read and follow label directions since instructions do vary. One product recommends mixing 4 teaspoons of Bt concentrate for every 1 gallon of water in a small garden sprayer. The larvae die after ingesting foliage containing the bacteria, so thoroughly cover foliage or needles so the spray makes direct contact with the feeding pests. Repeat applications every 14 days until you no longer spot bagworms feeding on your plants.
Treat With Permethrin
Spraying trees with a permethrin-based insecticide can offer some relief from bagworm infestations, but the ingredients will also kill off beneficial and pollinating insects along with the pests. As always, read the label's directions before mixing and spraying. One product recommends mixing 1/8 to 1/4 teaspoon of permethrin concentrate for every 1 gallon of water. Use a handheld garden sprayer to thoroughly cover the plant's foliage and stems so the larvae ingest the chemicals while feeding. Avoid spraying flowers or the blooms might turn brown. Spray in the late evening or early morning hours for optimal results. Repeat application every 14 days until the bagworms disappear.
In order to be effective, you must start treating bagworm-infested plants with insecticide as early as possible. Experts at the University of Nebraska-Lincoln Extension recommend spraying only the bags smaller than 1/2-inch long. Spraying larger bags won't provide very much pest control. Once the bagworms stop eating and start pupating inside of the bags, which typically occurs by late August, there's no point in spraying because the bacteria can't reach the pests.
Even nontoxic sprays can cause skin and eye irritation, so reduce your risk of exposure by wearing protective eye wear and clothing. No matter what insecticide you choose to use, only spray on calm days when no precipitation is expected in your area for at least 24 hours. Keep people and animals out of the treatment area until the solution dries.