Bagworms are the larval stage of a moth, identified by the protective, spindle-shaped bags the caterpillar constructs from silk and pieces of the host plant’s foliage. These distinctive structures are carried by the insect as it feeds. Bagworms are voracious defoliators, causing severe damage particularly to evergreen trees and shrubs like arborvitae, juniper, and pine. Heavy infestations lead to the permanent loss of foliage on conifers and can result in the death of the plant if left untreated.
The Critical Timing for Spray Application
Treatment effectiveness depends entirely on application timing, which must align with the pest’s life cycle. Eggs overwinter inside the female’s bag and hatch in late spring, typically from late May through June, depending on local climate. Young larvae are extremely small and begin feeding actively immediately after hatching. This period, when the larvae are small and actively ingesting plant material, is the only effective window for insecticide sprays.
Once the larvae mature and their protective bags become fully sealed and tough—usually by late summer or early fall—spraying is useless. The insecticide cannot penetrate the thick, silken bag, and the caterpillars stop feeding before pupating. Therefore, treatment must be applied while the larvae are still developing and moving, generally when the bags are less than an inch long. Missing this narrow window allows bagworms to continue feeding and causing damage, leaving eggs for a new infestation the following spring.
Biological and Low-Impact Spray Options
For a selective, environmentally conscious approach, the biological insecticide Bacillus thuringiensis subspecies kurstaki (Btk) is the preferred treatment. Btk is a naturally occurring soil bacterium that produces a protein toxin specific to the digestive system of caterpillars. The bagworm must ingest the treated foliage for Btk to activate in its gut, causing the caterpillar to stop feeding and die. Since it is a stomach poison, Btk has no effect on beneficial insects, pets, or humans, making it a low-impact choice.
The short residual effect of Btk means it breaks down rapidly in sunlight, often requiring reapplication to maintain efficacy. Because bagworm eggs hatch over an extended period, a second application seven to ten days after the first may be necessary to target newly emerged larvae. Spinosad is another low-impact option, derived from a soil-dwelling bacterium, acting as both a contact and stomach poison. Horticultural oils and neem oil also offer supplementary control by suffocating the tiny larvae, but they require direct and thorough coverage.
Conventional Chemical Control Options
When bagworm infestations are severe or the treatment window for smaller larvae has been missed, a conventional contact insecticide may be required. These synthetic options provide a quicker knockdown and are more effective against larger larvae (up to one inch long) that have grown out of the Btk-susceptible stage. Common active ingredients include pyrethroids such as lambda-cyhalothrin, bifenthrin, and cyfluthrin, as well as carbaryl. These broad-spectrum chemicals kill insects through direct contact and residual action on the plant surface.
These insecticides are broad-spectrum, meaning they can harm non-target insects, including pollinators and natural predators. Application must be timed for late spring or early summer when bagworms are feeding but before flowering plants attract beneficial insects. Thorough coverage is necessary, ensuring the spray reaches the interior of the plant canopy where larvae are often hidden. Always read the product label before mixing and applying any chemical spray. Safety precautions must be followed, including wearing appropriate protective gear and avoiding application during windy conditions to prevent chemical drift.