The bagworm (Thyridopteryx ephemeraeformis) is a common pest that targets a wide range of trees, including evergreens and deciduous species. It is most recognizable by the spindle-shaped, protective casing it constructs from silk and pieces of the host plant’s foliage. This camouflage allows the larva to feed and develop inside a secure shelter, expanding the casing as it grows throughout the season. Left unchecked, a severe infestation can cause significant defoliation, particularly on conifers like juniper and arborvitae, potentially leading to the death of the tree over multiple seasons. Effective removal requires understanding their life cycle and applying targeted control strategies.
Recognizing Bagworm Infestation and Timing Treatment
Successful bagworm management depends on proper timing, dictated by the pest’s annual life cycle. Infestations are confirmed by cone-shaped bags hanging from branches, camouflaged with needles or leaves. Noticeable defoliation, appearing as browning or thinning foliage, is the most obvious sign of feeding, especially on evergreens.
Bagworm eggs overwinter inside the female’s mature bag, protected from weather and most insecticides. Larvae hatch and emerge from late May through early July. This emergence period is the only time chemical treatments are effective, as the young larvae are exposed and actively feeding.
Larvae reach maturity, usually by mid-to-late August, securing their bags to a branch to pupate inside. Once the bag is sealed, the bagworm stops feeding, rendering topical insecticides useless. Treatment application in late spring or early summer, when larvae are small, is crucial for effective control.
Non-Chemical Control: Manual and Biological Methods
For smaller trees or light infestations, manually removing the bags during the non-feeding season is an effective control method. This should be done from late fall, after feeding has stopped, through early spring, before the eggs hatch. Physically removing the bags eliminates the overwintering egg masses that produce the next generation of pests.
It is crucial to destroy the removed bags completely, as simply dropping them on the ground allows the eggs to remain viable and potentially hatch. The bags should be collected and disposed of in a sealed container or immersed in a bucket of soapy water. Cutting the silk band used to attach the bag is also important, as this tough material can girdle the branch as the tree grows, causing long-term damage.
A biological control option involves the use of Bacillus thuringiensis subspecies kurstaki (Bt). Bt is a soil bacterium harmless to humans, pets, and beneficial insects like bees and ladybugs. This application is effective only when ingested, meaning it must be sprayed directly onto the foliage consumed by young, actively feeding bagworm larvae.
The bacteria produce a protein crystal that becomes toxic once dissolved in the alkaline gut of the caterpillar, causing the pest to stop feeding and die within a few days. For maximum efficacy, Bt must be applied when the larvae are small, typically within a few weeks of hatching. Repeat applications may be necessary to target larvae that hatch later or are “ballooned” in from neighboring plants via silk threads.
Strategic Use of Insecticides
When infestations are widespread or trees are too large for manual or biological control, chemical insecticides are an option. These products provide a more persistent residual effect on the foliage than biological options. Effective contact insecticides include active ingredients such as permethrin, carbaryl, and pyrethroids like lambda-cyhalothrin.
Sprays must be applied thoroughly, ensuring the insecticide reaches the highest branches and the underside of the foliage. Contact insecticides are only successful when bagworms are in their larval stage and actively consuming treated leaves or needles. Once the larva seals its bag to pupate in late summer, no topical spray will penetrate the protective casing.
Another option for larger trees is systemic insecticides, which are absorbed by the tree and move throughout its vascular system, providing internal protection. Products containing acephate can be applied as a soil drench or trunk injection, poisoning the foliage the bagworms consume. Systemic treatments offer a longer window of protection but should be reserved for severe cases or where thorough spray coverage is difficult.
When applying chemical control, follow all instructions on the product label to minimize environmental risk. To protect non-target insects, especially pollinators, sprays should be applied in the early morning or late evening when beneficial insect activity is minimal. Wearing protective gear, such as gloves and eye protection, is required during application.
Long-Term Monitoring and Tree Health
Controlling an active bagworm infestation is only part of management; long-term prevention is crucial for maintaining tree health. Stressed trees are more susceptible to pest damage, including bagworm attack. Maintaining tree vigor through proper horticultural practices helps strengthen the plant’s defenses.
This includes providing adequate moisture during drought and applying mulch around the base of the tree to regulate soil temperature and retain water. An annual inspection of susceptible host trees should be conducted in late winter or early spring before the new generation hatches. Early detection allows for easy manual removal and reduces the need for widespread chemical treatment later in the season.