A number of insects carry the common name of bagworm, but for North American gardeners it is Thyridopteryx ephemeraeformis that usually comes to mind. Known as the evergreen bagworm, or simply the bagworm, it is easily identified by the dry, bag-like cocoons these caterpillars create in the branches of trees and shrubs. Although the bags themselves are somewhat disfiguring, the real problem is the damage wreaked by caterpillars as they eat the foliage of more than 100 susceptible species. In the larval (caterpillar) stage, this insect commonly attacks a great many valuable landscape species, including arborvitae, red cedar, many juniper species, fir, maple, juneberry, buckeye, persimmon, ginkgo, honeylocust, larch, sweetgum, spruce, pine, sycamore, poplar, oak, locust, willow, and hemlock.
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.
What is the lifecycle of the bagworm?
The lifecycle of the evergreen bagworm is one of the more interesting of all insects. The female adult insect is a maggot-like, soft-bodied yellow-white worm that dwells within a bag she constructed while in her larval caterpillar stage. After laying 500 to 1000 eggs inside the bag, the female dies, and the eggs overwinter in the bag before hatching the following spring. In May and June, the larvae emerging from the egg as tiny caterpillars begin to leave the bag. Immediately, each one begins to build its own bag around the lower part of its body, leaving its head and legs free to move about the plant and feed voraciously on foliage. If a branch becomes completely defoliated, the larvae will crawl away, dragging the attached bags, to attack another branch, or move to a whole new plant. By mid-August the bagworm, now about 1 inch long, has completed its growth; it now attaches its bag to a limb, seals it, and begins the pupal transformation. By mid-September, the adult males emerge from the bags as black furry moths with feathery antennae. The transparent wings are about 1 inch long. The female transforms into a maggot-like worm inside the bag, then awaits a male moth to mate with her through an opening in the bag. It lays eggs, then dies to begin the next generation—without ever leaving the bag it first constructed as a larval caterpillar.
Can bagworms infest homes?
The bagworm species that infests outdoor trees can potentially make its way indoors if attached to a potted tree or another plant that is brought indoors for the winter, but this is not a pest that seeks warm indoor shelter. However, there is another type of bagworm, called the plaster bagworm (Phereoeca uterella), that is quite often found indoors. The larval stage of this small moth creates its own case made from sand, grit, and insect droppings—a case that is about the shape and size of a pumpkin seed. The caterpillar attaches this case to walls and ceilings as it enters the pupal phase. If you see these gritty, seed-like cases hanging around your home, they are likely plaster bagworms. This insect is closely related to the common clothes moth and is controlled in the same way. The primary food of the plaster bagworm is the silk strands of spider webs, so eliminating cobwebs is a common means of control.
Do bagworm moths feed on plants?
The moths of the evergreen bagworm are the adult male phase of the insect, and they exist only to find females and mate. They only live for a few days and do not eat at all during this time. The moths are not commonly seen, but you can sometimes spot one fluttering around light sources at night, usually in September.
How do bagworms travel from plant to plant?
A tiny caterpillar that hatches to find the host plant already defoliated sometimes will disperse itself by casting a strand of silk that catches the wind to transport it to a new location. It can take all summer for the caterpillars to reach their mature size, about 1 inch long, at which time they anchor their bags and enclose themselves to pupate.
Sadof, Clifford. Bagworms. Purdue Department of Entomology.