The bagworm is a destructive pest that poses a threat to landscape plants, particularly the dense foliage of arborvitae. These pests are difficult to spot until significant damage has occurred because their protective cases blend seamlessly with the host plant material. Arborvitae’s thick, evergreen needles provide both camouflage and a continuous food source, making it a prime target for infestation. Evergreens cannot quickly regenerate defoliated tissue, meaning bagworm damage can quickly lead to permanent branch dieback or the death of the entire plant. This guide provides a strategic approach to eradicate bagworms, focusing on the narrow window when they are most vulnerable.
Recognizing the Pest and Effective Timing
Identifying the bagworm pest itself is less useful than recognizing the spindle-shaped protective case it constructs, which is made of silk woven with bits of arborvitae foliage. These bags can range up to two inches long as the larva grows, and they are often mistaken for small pinecones or dead plant structures. The first noticeable sign of an active infestation is browning or thinning foliage, typically starting at the top of the tree. The timing of any control measure is important because the bagworm’s life cycle provides only a short window for effective intervention.
Bagworms overwinter as eggs, with each mature female bag containing hundreds of eggs that remain dormant through the cold season. The eggs begin to hatch into tiny, mobile larvae typically from late May to early June. This period, when the larvae are small and actively feeding outside their newly spun bags, is the only time chemical treatments are effective. Once the caterpillars mature and seal themselves within their larger, reinforced bags in mid-to-late summer, they are protected from insecticide applications.
Immediate Physical Removal Strategies
For small infestations or as a preventative measure during the dormant season, manual removal is the simplest and most effective non-chemical control method. This strategy is essential during the late fall, winter, and early spring before the overwintering eggs hatch. Using sharp pruning shears, carefully snip the silken thread attaching each bag to the arborvitae branch. It is important to remove the entire silk band, as leaving it wrapped around the branch can lead to girdling, which chokes off the flow of nutrients and water, causing branch death.
Once the bags are removed from the tree, they must be destroyed immediately, as simply dropping them on the ground will allow the eggs inside to hatch later. The most reliable disposal method is to submerge the bags in a container of soapy water for several days to drown the eggs and larvae. Alternatively, the removed bags can be sealed tightly in a plastic trash bag and disposed of with household waste. This physical removal process is the only way to eliminate the resistant eggs and is the first line of defense against future generations.
Targeted Spray Applications
For larger or taller infestations where manual removal is impractical, targeted spray applications provide control, but only when precisely timed. The spray must be applied immediately after the eggs have hatched and the larvae are small, which typically occurs in late May or throughout June. Because the larvae must ingest the product, thorough coverage of the entire arborvitae, especially the newest growth where they are feeding, is necessary to ensure success.
The preferred biological control agent is Bacillus thuringiensis subspecies Kurstaki (Bt), a naturally occurring bacterium effective only against young caterpillars. Bt must be consumed by the actively feeding larvae, where it paralyzes their digestive system, leading to death within a few days. This product has minimal impact on beneficial insects, such as pollinators and predators. To maximize ingestion, apply Bt late in the day or in the evening when bagworms are most actively feeding and when the product is less likely to be degraded by sunlight.
If the bagworms are discovered later in the season when they are larger or if the infestation is severe, a conventional insecticide may be necessary. Products containing active ingredients like permethrin, bifenthrin, or carbaryl are effective against more mature larvae. These synthetic insecticides are broad-spectrum, meaning they kill a wider range of insects, so they should be used with caution and strictly according to label instructions. Regardless of the product chosen, the dense, overlapping foliage of the arborvitae requires a high-pressure, high-volume application to ensure the spray penetrates to the interior branches and fully coats the needles.
Long-Term Health and Prevention
After successfully controlling an infestation, focusing on the arborvitae’s health is important to aid in its recovery from defoliation. Evergreens that have suffered damage will benefit from consistent watering, especially during dry periods, to reduce stress on the plant. Applying a balanced, slow-release fertilizer in the spring can provide the necessary nutrients to support new growth and help the tree compartmentalize the damaged areas. However, do not over-fertilize, as this can encourage weak growth that may attract other pests.
The most effective long-term prevention strategy is yearly monitoring, particularly during the late fall and winter months. Inspecting the trees when the bags are easily visible against the green foliage allows for the manual removal of overwintering egg sacs before they hatch. Regular inspection in late spring is also advised to catch the tiny, newly hatched larvae and their small bags, which provides the earliest opportunity for a targeted biological treatment. Maintaining the overall health of the arborvitae reduces its susceptibility to severe infestations.