Bagworms pose a significant threat to Arborvitae, a popular evergreen used for hedges and screening. These pests can completely defoliate conifers, and if an infestation is left untreated, it can lead to the death of the plant. Arborvitae are particularly vulnerable because, unlike deciduous plants, they cannot easily regrow consumed needles. Successful management requires timely monitoring and the correct application of control methods.
Identifying Bagworm Infestation and Timing
The most recognizable sign of an infestation is the presence of spindle-shaped bags hanging from the branches. These cases are constructed by the larvae using silk interwoven with foliage and bark from the host plant, providing effective camouflage. The bags start very small, often less than a quarter-inch long in early summer, but can grow to over two inches as the caterpillar matures.
Timing the treatment is the single most important factor for effective control, as insecticides are only effective when the bagworms are small and actively feeding outside the protective casing. Bagworm eggs, which overwinter inside the female bags, typically hatch from late May through June, depending on the local climate. Treatment applications must target this window when the newly hatched larvae are mobile and consuming foliage. Once the larvae are larger or have sealed themselves inside their mature bags in late summer, spraying becomes virtually useless.
Effective Chemical and Biological Controls
The appropriate spray material depends heavily on the size of the larvae and the severity of the infestation. Control options are generally divided into microbial biopesticides and synthetic chemical insecticides.
Biological Control
One highly effective option for young bagworms is the use of Bacillus thuringiensis (Bt) subspecies kurstaki. This soil bacterium must be ingested by the caterpillar to work, releasing a toxin that disrupts the insect’s digestive system. Because Bt only affects feeding caterpillars and is harmless to humans, pets, and beneficial insects like bees and ladybugs, it is considered a selective choice. For best results, Bt should be applied when the bags are still small, ideally less than one inch, and may require a second application ten to fourteen days later.
Another biological insecticide is Spinosad, which is derived from a naturally occurring soil organism and controls bagworms through both ingestion and contact. Spinosad is often recommended because it maintains effectiveness for a longer period than Bt and is less hazardous to many beneficial insects than broad-spectrum chemicals. However, when larvae are older or the infestation is severe, a more aggressive approach may be necessary.
Chemical Control
For infestations detected later in the season or when the caterpillars have grown larger, synthetic insecticides are often utilized. Products containing active ingredients like Permethrin, Bifenthrin, or Lambda-cyhalothrin are effective contact and residual options. These compounds belong to the pyrethroid class and provide a quick knockdown effect, killing the pest upon contact or when ingested.
Carbaryl is another broad-spectrum insecticide effective against larger bagworms. Because synthetic insecticides are broad-spectrum, they can impact non-target insects, including pollinators, so their use should be reserved for cases where biological controls are insufficient. Always ensure the product label specifically lists Arborvitae as a plant it can be safely used on, as some formulations may cause plant damage.
Non-Chemical Management and Prevention
Manual removal is the most reliable non-chemical method, particularly effective during the fall and winter months. Since the eggs overwinter inside the female bags, physically removing and destroying these cocoons before the eggs hatch in late spring eliminates the next generation. Each female bag can contain between 500 and 1,000 eggs, making this a high-impact prevention strategy.
The removed bags should not be dropped on the ground beneath the plant, as the eggs inside can still hatch and the larvae can crawl back onto the Arborvitae. Instead, place the bags in a sealed trash bag for disposal or submerge them in a bucket of soapy water to ensure the eggs are destroyed. For tall Arborvitae where handpicking the highest bags is impractical, a strong jet of water from a garden hose can dislodge the bags for collection. Maintaining the overall health of the Arborvitae through proper watering and avoiding plant stress can help the plant better withstand and recover from pest pressure.
Safe and Proper Application Techniques
Following proper application techniques is necessary to ensure effectiveness and safety. Always read the product label before mixing or applying any material, as it contains specific instructions for dilution rates and approved plants. Personal protective equipment (PPE), such as gloves, long sleeves, eye protection, and often a respirator, should be worn to minimize exposure to the concentrate and spray mist.
Thorough coverage of the Arborvitae foliage is paramount, especially since bagworms commonly start feeding at the top of the plant. The spray must reach the entire plant, including the inner canopy and the undersides of the branches, to ensure the feeding larvae contact the material. Adding a spreader-sticker, also known as a surfactant or adjuvant, to the spray mixture can help the product adhere better to the waxy needles, improving efficacy. To protect beneficial insects, especially pollinators, spray applications should be performed during calm, cooler periods, typically in the early morning or late evening hours.