Bagworms (Theridopteryx ephemeraeformis) are moth larvae that feed on various deciduous and evergreen bushes. They emerge in late spring to early summer, spinning protective cases of silk and foliage around themselves. The spindle-shaped bags reach about 2 inches in length by summer's end, when they quit feeding and start pupating. Besides their trademark bags, you know you have bagworms on your shrubs when evergreen needles turn brown on the tips or deciduous leaves suddenly have little holes. Without intervention, affected bushes suffer stunted growth, loss of vigor, defoliation and even death. You can get rid of bagworms using various control methods.
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Control small bagworm populations by handpicking bags or snipping them off the bush with scissors. You must remove bags from winter through early spring before the eggs start hatching in order to control the pests. Search the entire bush for the cases, because every one you miss can lead to 1,000 new bagworm pests. Toss plucked bags in a covered trash can, crush them underfoot or drop them into a container of soapy water. Don't just drop the bags to the ground, because the larvae can hatch and find a host shrub to feed upon. Remember to cut off the silken threads the pests use to attach the bags to branches or twigs. If you don't, that thread can eventually girdle and even kill the wood tissue.
Several insect predators, including parasitic wasps and tachinid flies, eat bagworm eggs, larvae and pupae. Help attract these beneficial insects by planting annual flowers, such as asters (Aster spp.), zinnias (Zinnia spp.) and parsley (Petroselinum crispum), near bagworm-infested bushes. Be careful using broad-spectrum insecticides if you want to promote natural predators in your landscape. The chemicals will undoubtedly wipe out the beneficial insects along with the bagworm pests.
Bagworms are also a treat for various birds, particularly finches and sparrows. Keep your fine-feathered friends around by placing birdhouses, bird feeders and bird baths around your yard. Planting seed- or fruit-bearing plants around your landscape provides birds with extra food and nesting materials.
Large bagworm populations are often easier to treat using an insecticide containing Bacillus thuringiensis var. kurstaki. Btk is a natural soil-borne bacterium that causes moth larvae to starve to death, but won't affect beneficial insects, earthworms, people or animals. For the bacterium to work effectively, you must dust or spray affected plants when the bags are still smaller than 1/2-inch long, typically by late spring or early summer.
For small shrubs, consider coating the foliage with a thin, even layer of Btk dust. For taller bushes, make a spray solution by following the manufacturer's directions. One Btk product recommends 4 teaspoons of product for every 1 gallon of water. Use a handheld garden sprayer to completely coat all of the foliage, including the undersides of leaves. The bagworms must consume treated foliage for the product to work. Repeat treatments every five to seven days while bagworms remain active.
If you don't catch the bagworms early enough for Btk sprays to work, a pyrethroid-based pesticide might be in order. Pyrethroids control both small and large caterpillars, but don't wait too long -- by late summer, the pests have typically stopped feeding and are pupating inside of their little bags. Carefully read and follow the instructions on the product's label before using. One insecticide recommends mixing 1 tablespoon of concentrate for every gallon of water. You must penetrate the bush and cover the foliage, including the undersides, so the feeding bagworms eat the treated leaves. Check for active bagworms in one to two weeks, repeating the application, if necessary.
Always read the label before use, making sure both bagworms and the plant you plan to treat are listed. Even natural insecticides can irritate your eyes and skin with direct contact. Reduce your risk of exposure by donning safety glasses and protective clothing when mixing and spraying any insecticidal solution. Keep people and pets away from treated bushes until the solution dries, typically about 4 hours. Apply sprays when rain isn't expected for at least 24 hours and on days with little to no wind. Pyrethroids are toxic to honeybees, so spray in the early morning or late evening when the pollinators aren't present.
Bagworms feed on about 130 different plant species, but are most problematic on evergreens, especially junipers (Juniperus spp.), pine (Pinus spp.), spruce (Picea spp.) and arborvitae (Platycladus spp., Thuja spp.). Depending on the variety, junipers are hardy in U.S. Department of Agriculture plant hardiness zones 3 through 10, spruce is hardy in USDA zones 2-8, and arborvitae is hardy in USDA zones 2 through 10. Because of this, the pest is often called the "evergreen bagworm." Keep in mind that the pests don't always stay on their initial host shrub. After defoliating a bush, the larvae will migrate to another plant and start munching.