Bagworm larvae construct a protective silken case that camouflages them as they feed. These destructive pests pose a particular threat to cedar trees and other evergreens because conifers do not readily replace lost foliage. Significant defoliation can cause permanent damage, leading to a sparser appearance or even the death of the tree if an infestation is left unchecked. Effective management requires recognizing the signs early and applying the correct control method at the precise moment in the pest’s life cycle.
Identifying Bagworm Infestation
The most visible indication of an infestation is the presence of the insect’s distinctive, cone-shaped silk shelter hanging from the branches. Larvae create these “bags” by incorporating bits of the cedar’s foliage and bark into the silk, which allows the case to blend in with the needles. This camouflage often causes them to be mistaken for small pinecones or dried foliage tips.
These spindle-shaped bags range from one-half inch up to two inches long when the larvae mature in late summer. Damage appears as browning tips, stripped needles, or localized patches of defoliation. Since the female bagworm cannot fly, local populations build up quickly, often concentrating heavy damage in one area before spreading. Look for these bags hanging vertically from the branches, as they contain the overwintering eggs for the next generation.
Mechanical Control: Physical Removal
Hand-picking the bags is the most effective, non-chemical control method, especially during the dormant season (late fall through early spring). Removing the bags before the eggs hatch in late May or early June eliminates the next generation of pests entirely. This method is useful for smaller trees or when the infestation is localized and within easy reach.
Use sharp pruning shears or scissors to cut the silken band securing the bag to the branch. Simply pulling the bag off can tear the bark or leave the silk band, which may girdle and kill the branch as the tree grows around the constricting silk. Once removed, the bags must be destroyed to prevent the 300 to 1,000 eggs inside from hatching.
Bag Destruction Methods
- Submerge the bags in a container of soapy water for several days.
- Seal the bags tightly in a plastic bag and discard them in the trash.
Targeted Treatment Methods
Chemical and biological treatments are only effective when the bagworms are small, actively feeding, and exposed to the treated foliage. This window occurs after the eggs hatch, typically from late May through the end of June. Once the larvae seal their bags shut for pupation in late summer, no spray can penetrate the protective silk to kill the insect inside.
The favored option for control is the biological insecticide Bacillus thuringiensis subspecies kurstaki, commonly referred to as B.t. This naturally occurring bacterium works as a stomach poison, requiring the caterpillar to ingest the treated foliage for the toxin to activate in its alkaline gut. B.t. is highly specific to caterpillars and does not harm beneficial insects, pets, or humans, making it a desirable treatment for home landscapes.
For B.t. to be successful, it must be applied when the larvae are still very small (less than one-half inch long) and actively chewing on the cedar needles. A thorough application is needed to coat all the foliage the young bagworms will consume. If the infestation is widespread or severe, broader-spectrum chemical insecticides containing active ingredients like permethrin or carbaryl may be necessary. These chemical options provide a quicker knockdown and may be more effective against slightly larger larvae. However, because they are broad-spectrum, they must be used with caution to minimize impact on beneficial species.