In this larval stage, the caterpillars can gradually destroy a tree or shrub, first leaving it partly defoliated and unsightly, then gradually defoliating it so completely that it weakens the tree enough to kill it. The most visible early symptom of infestation by the Thyridopteryx ephemeraeformis bagworm is the presence of many brown, bag-like cases hanging from the limbs like Christmas tree ornaments. These cocoon bags are generally constructed by the caterpillars from pieces of dried stem and leaves, and they may look like deformed pine cones, up to 2 1/2 inches long.
4 Ways to Get Rid of Bagworms
At one time, the recommended advice for getting rid of a bagworm infestation (the only advice, really) was to pluck each bag/cocoon out of the tree or shrub, one at a time, to interrupt the insect’s lifecycle. That is still a viable method, but this can be quite difficult with a large landscape tree that may have many dozens, perhaps even hundreds, of bags hanging from its limbs. Fortunately, there are other methods to control this unusual insect, in addition to manual removal.
Remove the Bags by Hand
For smaller trees and shrubs, it is quite possible to remove the bagworm’s bags by simply plucking them from the tree and destroying them. This purely organic approach may require you to hire a specialist to reach tall branches with ladders or equipment. Ideally, the bags should be removed from late fall to spring (roughly October to March) to ensure you are removing as many eggs as possible.
Encourage Natural Predators
Light infestations of bagworms on large, healthy trees are often not much of a problem, as birds and some insect predators will feast on the young caterpillars. Woodpeckers may even break apart the bags to eat the females or eggs inside. Sparrows are another well-known predator of bagworms. You can encourage this kind of predation by making your landscape bird-friendly and avoiding the use of pesticides that kill helpful insects. New studies at the University of Illinois at Urbana-Champaign show promise that planting members of the aster family near and/or around the susceptible tree will attract a major natural predator of bagworm, the ichneumonid wasp. These tiny members of the wasp family parasitize other insects by laying their eggs inside prey insects, but the wasps themselves are harmless to humans. The aster family includes many species with daisy-like blossoms. The UIUC study showed effectiveness with shasta daisy, Newfoundland aster, and treasure flower (Gazania rigens), but other members of the aster family will also draw parasitic wasps.
Spray With Bt (Bacillus thuringiensis)
Bt is a naturally occurring soil bacterium that causes the bagworm caterpillars to become sick, stop feeding, and then die. The best time to spray with Bt is when the young worms are hatching and emerging from the bags, usually in late May or early June. Check with your local Extension office to learn the best timing, as well as the recommended strain of bacteria to apply.
Use Chemical Controls
The use of synthetic chemical pesticides should be reserved for serious bagworm infestations on valuable landscape trees. Recommended chemicals to control bagworms include acephate (Orthene), cyfluthrin, and spinosad, applied as sprays. Of these three, spinosad has the lowest toxicity to humans and other mammals, but all three are highly toxic to bees and other helpful insects. If you must apply a chemical control, try to do so on a windless, dry day so that the product can dry quickly and not drift on the breeze.
What Causes Bagworms?
Evergreen bagworms tend to favor hot, humid conditions, making them a more serious problem in the U.S. South and Southeast, though they can be found across the entire eastern U.S. Bagworms will usually attach weakened trees, so keeping landscape plants healthy is a good preventive measure. Bagworms often are a bigger problem in agricultural areas where pesticide use is high, causing predatory insects to be killed off.
How to Prevent Bagworms
Keep your plants healthy, and inspect the branches regularly for the small bags that indicate a building infestation. Removing them by hand will usually prevent a serious infestation. When buying sapling trees and shrubs, inspect the branches carefully to make sure that you’re not bringing infested plants into your landscape. Limiting the use of pesticides around your yard will help foster natural predators that keep bagworms from becoming a serious problem.
Bagworms vs. Tent Caterpillars vs. Web Worms
The highly damaging bagworms are sometimes confused with other types of moth insects that construct sheltering structures in plants. The tent caterpillar (Malacosoma spp.) comprises several species that construct web-like tents in crotches of branches, which serve as shelters for the caterpillars. Some species, such as the forest tent caterpillar, can be quite devastating, defoliating large sections of forest land. Other tent caterpillars, however, do relatively minor damage when found on healthy trees. Tent worms can be managed using the same techniques used for bagworms. Fall webworms (Hyphantria cunea) form web structures similar to tent caterpillars, but unlike tent caterpillars, these insects are largely harmless to most trees and shrubs. Webworms typically form fairly loosely woven webs that enclose branches, while the web of the tent caterpillar is a thick construction found in the forks and crotches of trees. Webworms are usually found in summer and fall, while tent caterpillars appear in spring.
