Bagworms are the larval stage of moths (Psychidae family) and pose a significant threat to landscape plants, especially evergreen trees and shrubs. These pests cause rapid and severe defoliation, which can lead to the decline or death of host plants if not managed promptly. Effective control requires a combination of physical removal and precisely timed treatments. Understanding the bagworm’s life cycle and recognizing their windows of vulnerability are essential for protecting your valuable landscape.
Recognizing Bagworm Stages and Damage
The most obvious sign of an infestation is the characteristic spindle-shaped bag hanging from plant branches. Larvae construct these protective cases using silk threads, camouflaged with bits of foliage, bark, and debris from the host plant. The bags start small (around one-eighth of an inch long) but can grow up to two inches by the end of the summer feeding period.
Bagworms overwinter as hundreds of eggs contained inside the female’s bag from the previous season. Larvae typically hatch in late spring or early summer, generally from late May through early June, and immediately begin creating their own tiny bags. They feed actively until late summer, which is when they cause the most noticeable damage.
The resulting damage manifests as browning or thinning foliage, especially on conifers like arborvitae, juniper, and spruce. Evergreens are particularly susceptible to irreversible harm because they do not readily regrow their needles after defoliation. If a bagworm infestation is left unchecked, the complete stripping of foliage can easily lead to the death of the host plant within a single season.
Manual Removal and Physical Control Methods
Hand-picking the bags is a straightforward and highly effective method for controlling small, localized infestations. This physical control is best performed during the dormant season (late fall through early spring), as the bags contain overwintering eggs. Removing the bags before the eggs hatch in late spring prevents hundreds of larvae from emerging to start a new generation.
When removing the bags, it is important to clip the silk band that attaches the bag to the branch using a sharp shear or knife. Leaving the silk band behind can cause the branch to be girdled as it grows, leading to branch dieback over time. Once removed, the bags must be destroyed rather than simply dropped on the ground or placed in a compost pile.
To destroy the bags, seal them in an airtight plastic bag or immerse them in a container of soapy water to ensure the eggs and larvae are killed. This approach is the least toxic option and is the primary strategy for managing infestations on small trees and shrubs that are easily accessible. Regularly inspecting susceptible plants and removing the bags throughout the dormant season provides a strong foundation for population management.
Optimal Timing for Treatment Application
Timing is the single most important factor for the successful use of chemical or biological treatments against bagworms. Insecticides are ineffective against overwintering eggs because the tough, silken bag completely protects them. Treatments must be applied when the larvae are actively feeding and are still small enough to be vulnerable.
The window of vulnerability begins in late May and extends through July, depending on the specific region. Homeowners should begin monitoring plants closely in late spring for signs of newly hatched, tiny larvae. These young bagworms are often less than half an inch long and have only just begun constructing their cases, making them highly susceptible to control products.
The effectiveness of any spray application decreases significantly as the season progresses and the larvae mature. By mid-August, bagworms stop feeding and securely seal their bags to pupate, rendering insecticide application ineffective. Therefore, the goal is to target the larvae when they are small, mobile, and consuming large amounts of treated foliage, typically from late May through late June.
For the best results, a follow-up application is often necessary, applied about 10 to 14 days after the initial treatment. This ensures that any larvae that hatched after the first application are also exposed. Missing this narrow window means waiting until the next spring to manage the population, allowing the current generation to lay hundreds of eggs.
Effective Chemical and Biological Treatments
When manual removal is impractical due to the size of the plant or the extent of the infestation, targeted sprays offer the most reliable control. The choice of treatment depends largely on the maturity of the larvae and the desire to protect beneficial insects. Biological control agents are preferred for young larvae due to their selective nature.
The biological insecticide Bacillus thuringiensis (Bt, subspecies kurstaki) is highly effective against young bagworm larvae. Bt is a naturally occurring soil bacterium that must be ingested by the caterpillar, where it disrupts the digestive system. Because of this mechanism, it is only effective when larvae are actively feeding and is safe for humans, pets, and beneficial pollinators.
Another biological option is spinosad, which is derived from soil bacteria and works through both ingestion and contact. Spinosad can provide a slightly longer window of control than Bt. However, both biological options require thorough coverage of the foliage so the larvae consume the material, and multiple applications are typically needed to manage the entire hatching period.
For larger, more mature larvae or heavy infestations, synthetic chemical insecticides may be necessary. Products containing active ingredients like permethrin or carbaryl are broad-spectrum and provide a faster knockdown of the pest. These synthetic pyrethroids persist longer on the plant surface, but their use should be limited to infested areas to minimize impact on non-target insects. A thorough application that completely covers the foliage is necessary because the larvae must crawl over and ingest the treated material.