Bagworms are destructive pests that can rapidly defoliate trees and shrubs, particularly evergreens like arborvitae, juniper, and spruce. These pests are the larval stage of a moth, known for constructing a unique, spindle-shaped protective case from silk and fragments of the host plant foliage. Effective management requires a precise, multi-step approach focusing on the insect’s vulnerable life stage, combining mechanical removal and correctly timed chemical or biological treatments.
Assessing the Infestation and Initial Steps
Control begins with correctly identifying the pest and assessing the extent of the damage to the host plant. Bagworms create telltale bags, which begin small but can grow to over an inch and a half long, often camouflaged with needles and leaves from the plant. Heavy feeding causes browning, thinning foliage, and defoliation, which can kill evergreens since they do not easily regenerate lost needles.
If an infestation is discovered in late summer, fall, or winter, spraying is no longer an effective control measure. The larvae inside these mature bags are protected by the dense silk and plant material, making them nearly impervious to insecticide penetration. Furthermore, by late August, the larvae stop feeding and prepare to pupate, meaning they will not ingest any treated foliage.
For dormant, mature bags, manual removal is the appropriate action, especially for smaller trees and shrubs. Each female bag contains hundreds of eggs that will overwinter and hatch the following spring, so removing and destroying the bags significantly reduces the next generation’s population. Bags should be snipped off the branch, as the silk band they use to attach can girdle and kill the branch as it grows. The removed bags must be destroyed by crushing or drowning them in soapy water, not simply discarded, which would allow surviving larvae or hatching eggs to return to the host plant.
Timing the Application for Maximum Effectiveness
Timing is the single most important factor for successful spraying treatment against bagworms. Application is only effective after the eggs have hatched and the tiny larvae emerge to begin feeding on the foliage. This period typically occurs from late May through the end of June, depending on local climate and temperatures.
The newly hatched larvae are minuscule, often no larger than a pencil tip, and are highly vulnerable because their protective bags are still very small. Spraying during this early stage ensures the insecticide contacts the larvae or is ingested by them as they feed on the treated foliage. Delaying treatment past early July drastically reduces efficacy because the larvae quickly increase in size, reinforcing their bags with more silk and plant material.
The optimal window for the first spray often aligns with the blooming of certain indicator plants, such as the catalpa tree or Japanese tree lilac. If treatment is necessary, applying a second treatment ten days to two weeks after the first is recommended to cover any late-hatching larvae. Once the bags reach about one inch in length, chemical control is significantly less reliable and generally not recommended.
Selecting the Appropriate Treatment
The biological control agent Bacillus thuringiensis (Bt) is a highly selective, reduced-risk microbial insecticide, and is one of the primary options for effective bagworm control. Bt must be ingested by the caterpillar, where it releases a protein toxin that disrupts the insect’s digestive system, leading to the cessation of feeding and eventual death. Bt is most effective against very young larvae, ideally when bags are less than half an inch long, and poses minimal risk to beneficial insects, pets, or humans. Thorough application to all feeding foliage is crucial since it must be eaten.
Another reduced-risk option is Spinosad, derived from a naturally occurring soil bacterium, which works by both contact and ingestion. Spinosad can offer better control than Bt for slightly older or larger larvae, though it remains most effective on smaller pests.
Chemical Treatments
For larger larvae or when a faster knockdown is needed, chemical insecticides may be used. Products containing pyrethroids, such as bifenthrin or permethrin, act as contact and stomach poisons. These insecticides often provide a longer residual effect and a quicker kill rate than biological alternatives. However, they are broad-spectrum, meaning they can harm non-target insects, including beneficial predators and pollinators. When using any chemical agent, it is a legal requirement to carefully read and follow all label instructions, as the label specifies the correct application rate and safety precautions.
Spraying Techniques and Coverage
Regardless of the product chosen, the success of the treatment depends heavily on achieving complete and thorough coverage of the infested plant material. Bagworms are small and often hide inside the foliage, requiring the spray to reach all surfaces where they are feeding. The application should be directed to the entire plant canopy, ensuring the tops and undersides of leaves or needles are coated.
It is particularly important to saturate the interior of the plant, not just the easily visible outer layer of the foliage. For large trees, a simple pump sprayer may not be sufficient to reach the upper branches, making a high-pressure sprayer or professional equipment necessary to penetrate the dense canopy. Preparing the solution involves accurately mixing the product concentrate with water according to the label’s specifications, preventing either a diluted, ineffective solution or an overly concentrated one that could damage the plant.
Safety during application is paramount; one must wear appropriate personal protective equipment, such as gloves, eye protection, and long sleeves, as directed by the product label. Application should be performed on a calm day to prevent spray drift, which could contaminate non-target plants, bodies of water, or adjacent properties. Wind speeds should be low, and the application should be done when no rain is expected for at least 24 hours to allow the spray to dry and adhere to the foliage effectively.