The bagworm (Thyridopteryx ephemeraeformis) is a moth caterpillar that threatens evergreen trees and shrubs. This insect constructs a distinctive, spindle-shaped protective case from silk and pieces of the host plant’s foliage. Larvae feed on the needles, causing defoliation that can lead to branch dieback or plant death, especially in conifers like arborvitae, cedar, and juniper. Effective control requires understanding the pest’s life cycle and applying the correct removal technique at the appropriate time. Taking action against this pest protects evergreens from long-term harm.
Identifying the Pest and Understanding Its Vulnerable Stages
Bagworms are easiest to identify by the bags they create, which are camouflaged with evergreen foliage, allowing them to blend with the plant. These bags start small but can grow over two inches long by the end of summer, securely attached to the host plant’s twigs with tough silken threads. Male moths eventually emerge from their bags, but the female remains inside, wingless, where she lays a large clutch of eggs, sometimes numbering up to 1,000.
The eggs overwinter inside the sealed bag, making the insect resistant to chemical treatments during the dormant season. The vulnerability window opens when eggs hatch in late spring or early summer, typically from late May through late June, depending on the climate. Newly hatched larvae are tiny and immediately begin constructing small bags as they feed on the foliage. This feeding stage, before the larvae become sealed within their protective cases, is the only time chemical treatments are effective.
Non-Chemical Removal: Manual and Physical Methods
Hand-picking the bags is the most reliable, non-chemical method for bagworm control and is effective year-round. This practice is especially important from fall through early spring, as bags removed during this time contain the overwintering eggs for the next generation. Physically removing the bags before the larvae hatch prevents hundreds of potential pests from emerging onto the evergreen.
To remove the bags, use pruners to snip the silk band that attaches the bag securely to the branch, ensuring the entire bag is detached. Do not drop the removed bags onto the ground, as the protected eggs inside can still hatch. The bags must be disposed of securely, either by sealing them tightly in a plastic bag and placing them in the trash, or by immersing them in soapy water for several days to ensure the eggs are killed.
Targeted Chemical Treatments and Application Timing
Chemical control should be reserved for moderate to severe infestations where hand-picking is impractical due to the evergreen’s size or the number of bags. The timing of application is primary; insecticides are only effective immediately after the larvae hatch while they are actively feeding on the foliage. This window usually falls between late May and early July, but monitoring the tree for the emergence of newly hatched caterpillars provides the best timing.
The least-impactful chemical option is the biological insecticide Bacillus thuringiensis (Bt), a naturally occurring bacterium that targets only caterpillars. Bt must be ingested by the bagworm to be effective, requiring thorough application to the foliage the larvae are actively eating. It works best on very young larvae, and because Bt degrades quickly, repeat applications spaced seven to fourteen days apart may be necessary.
For more persistent control or when larvae are slightly larger, conventional insecticides containing active ingredients like Spinosad offer a strong alternative. Spinosad is derived from a soil bacterium and is highly effective against chewing insects, working both upon contact and through ingestion. Other conventional contact insecticides, such as those with permethrin or bifenthrin, can also be used if labeled for bagworms.
Thorough coverage is necessary because the larvae hide and feed deep within the evergreen’s canopy. A hard spray should be directed to the inner parts of the tree to ensure the insecticide reaches the newly formed bags and the foliage being consumed. Adding a horticultural spreader-sticker product to the spray solution can help the treatment adhere to the waxy needles of the evergreen, improving effectiveness.
Long-Term Recovery and Prevention
After a successful treatment, the evergreen needs help recovering from the defoliation and stress caused by the bagworms. Providing good cultural care is important for the tree’s long-term health and ability to replace damaged foliage. This includes ensuring the evergreen receives adequate water, especially during dry periods, and applying a light, balanced fertilizer if the soil nutrients are low.
Long-term prevention relies heavily on annual monitoring to catch new infestations before they become established. Because the female bagworm cannot fly, local populations can build up rapidly if even a single egg-filled bag is missed. Inspecting evergreens in late winter or early spring is advised, specifically looking for any remaining bags from the previous season. Removing and destroying these bags is the most effective preventative measure against a reinfestation.