The bagworm is a highly destructive pest and is the larval stage of a type of moth from the family Psychidae. These insects are named for the protective, spindle-shaped silk casing they construct and carry with them throughout their feeding stage. This portable shelter, festooned with bits of the host plant’s leaves or needles, allows the bagworm to remain camouflaged as it feeds on garden and landscape plants. The presence of these unique cases signals a common infestation that can lead to significant damage if not properly managed.
Physical Characteristics and Identification
The most distinctive feature of the bagworm is its protective case, which is constructed from silk and interwoven with plant debris like twigs, bark, and foliage fragments. This bag can grow from a tiny quarter-inch size upon hatching to a length of 1.5 to 2 inches by late summer as the larva matures inside it. The materials used in construction provide exceptional camouflage, making the bag resemble a small pinecone or a natural part of the plant until the infestation is severe.
The larva is a grayish-brown caterpillar that extends its head and thoracic legs from the bottom opening of the bag to feed and move along the host plant. Adult bagworm moths show a dramatic difference between sexes: the male emerges from the bag as a small, sooty-black moth with transparent wings and a hairy body. The female remains inside her bag, never developing wings, legs, or mouthparts, appearing as a soft, grub-like, wingless creature.
The Annual Life Cycle
The bagworm cycle begins with eggs that overwinter inside the tough, silk-lined bags, often numbering between 500 and 1,000 per female. Hatching typically occurs in late spring to early summer, generally from late May through mid-June, depending on the region’s climate. The newly emerged larvae, which are tiny and mobile, immediately begin to spin their own miniature, conical bags and start feeding on the host plant’s foliage.
Throughout the summer, the larval stage is the active feeding and growth phase, during which the caterpillar enlarges its protective bag to accommodate its increasing size. By late August or early September, the mature larva permanently anchors its bag to a twig with a strong band of silk and then enters the pupal stage inside the case. Adult males emerge shortly after pupation to fly and mate with the wingless females, which remain sequestered inside their bags. The fertilized female then lays her eggs within her pupal case inside the bag before she dies, allowing the eggs to successfully overwinter for the next generation.
Impact on Landscape Plants
Bagworms are considered pests because their larval feeding causes significant defoliation of host plants, which can lead to branch dieback and even plant death. The caterpillars use chewing mouthparts to consume the leaves and needles, often leaving behind only the larger veins on broad-leafed plants. This feeding activity is particularly damaging to evergreens, such as arborvitae, juniper, cedar, and spruce, because they cannot easily regrow lost foliage.
Repeated or severe infestations on conifers cause the foliage to thin out and turn a brown or scorched color, a symptom known as “bronzing,” which results in permanent damage. While deciduous trees like maple, locust, and sycamore are also hosts, they are more resilient and can often refoliate after a heavy attack. However, the silk used to attach the bags to the twigs can sometimes girdle the branches as they grow, causing additional localized damage.
Effective Management Strategies
Effective control relies on precise timing that targets the vulnerable larval stage or the overwintering eggs. The simplest non-chemical method is physical removal: hand-picking the bags from the plants during the dormant season, typically from late fall through early spring. Removing and destroying these bags before the eggs hatch in May eliminates the source of the next generation.
For larger infestations where hand-picking is impractical, chemical control can be highly effective, but timing is paramount. Insecticides must be applied immediately after the eggs hatch in late May or early June when the larvae are small, mobile, and actively feeding outside the full protection of their bags. Products containing Bacillus thuringiensis (Bt), a naturally occurring bacterium, are most effective against these young caterpillars because it must be ingested to disrupt their digestive system. Once the bagworms grow larger, typically by mid-July or August, they are much harder to kill, and chemical sprays become ineffective once the larvae stop feeding and seal their bags for pupation.
The Annual Life Cycle
The bagworm cycle begins with eggs that overwinter inside the tough, silk-lined bags, often numbering between 500 and 1,000 per female. Hatching typically occurs in late spring to early summer, generally from late May through mid-June, depending on the region’s climate. The newly emerged larvae, which are tiny and mobile, immediately begin to spin their own miniature, conical bags and start feeding on the host plant’s foliage.
Throughout the summer, the larval stage is the active feeding and growth phase, during which the caterpillar enlarges its protective bag to accommodate its increasing size. By late August or early September, the mature larva permanently anchors its bag to a twig with a strong band of silk and then enters the pupal stage inside the case. Adult males emerge shortly after pupation to fly and mate with the wingless females, which remain sequestered inside their bags. The fertilized female then lays her eggs within her pupal case inside the bag before she dies, allowing the eggs to successfully overwinter for the next generation.
Impact on Landscape Plants
Bagworms are considered pests because their larval feeding causes significant defoliation of host plants, which can lead to branch dieback and even plant death. The caterpillars use chewing mouthparts to consume the leaves and needles, often leaving behind only the larger veins on broad-leafed plants. This feeding activity is particularly damaging to evergreens, such as arborvitae, juniper, cedar, and spruce, because they cannot easily regrow lost foliage.
Repeated or severe infestations on conifers cause the foliage to thin out and turn a brown or scorched color, a symptom known as “bronzing,” which results in permanent damage. While deciduous trees like maple, locust, and sycamore are also hosts, they are more resilient and can often refoliate after a heavy attack. The silk used to attach the bags to the twigs can sometimes girdle the branches as they grow, causing additional localized damage.
Effective Management Strategies
Effective control relies on precise timing that targets the vulnerable larval stage or the overwintering eggs. The simplest non-chemical method is physical removal: hand-picking the bags from the plants during the dormant season, typically from late fall through early spring. Removing and destroying these bags before the eggs hatch in May eliminates the source of the next generation.
For larger infestations where hand-picking is impractical, chemical control can be highly effective, but timing is paramount. Insecticides must be applied immediately after the eggs hatch in late May or early June when the larvae are small, mobile, and actively feeding outside the full protection of their bags. Products containing Bacillus thuringiensis (Bt), a naturally occurring bacterium, are most effective against these young caterpillars because it must be ingested to disrupt their digestive system. Once the bagworms grow larger, typically by mid-July or August, they are much harder to kill, and chemical sprays become ineffective once the larvae stop feeding and seal their bags for pupation.