The common bagworm, Thyridopteryx ephemeraeformis, is a moth caterpillar that poses a significant threat to ornamental trees and shrubs across the eastern half of the United States. This species is a destructive landscaping pest, particularly in urban and suburban environments. Infestations lead to severe defoliation, which compromises the health and structure of susceptible plants. Understanding the pest’s feeding habits is the first step in protecting landscape features from damage.
What Are Bagworms and How Do They Damage Trees
The pest is most easily recognized by the protective case the larva constructs, a unique, spindle-shaped bag woven from silk and camouflaged with fragments of the host plant’s foliage and twigs. These bags start small when the larvae first hatch but can grow to a size of one-and-a-half to two-and-a-half inches long as the caterpillar matures. The camouflage is so effective that the bags often blend seamlessly with the plant material, making early detection difficult.
Damage begins shortly after the eggs hatch, typically from late May into early June, when the tiny larvae emerge and immediately begin feeding and creating their individual shelters. The larvae remain within these bags for their entire lives, extending only their heads and front legs to move and consume foliage. They continuously strip the needles or leaves from the host plant throughout the summer to fuel their growth.
This continuous feeding leads to defoliation, giving heavily infested trees a browned or scorched appearance by mid-to-late summer. The consequence of this damage is far more severe for certain types of plants, particularly conifers. Evergreens that are completely stripped of their needles are unable to regenerate that foliage and will often decline or die entirely. Damage on these plants is permanent, necessitating timely intervention.
The Most Susceptible Host Trees
Bagworms are capable of feeding on over 120 different species of plants, encompassing both evergreen and deciduous varieties. However, their preference and the resulting damage severity are highly dependent on the host’s ability to recover. Conifers and other evergreens are the primary targets and suffer the most devastating, often fatal, outcomes from an infestation.
High-risk primary hosts include Arborvitae, Juniper, Cypress, Cedar, Pine, Hemlock, and Spruce. The permanent nature of conifer needles means that once the foliage is consumed, the tree cannot readily replace it during the growing season. This lack of recovery potential means that even a single season of heavy feeding can lead to branch dieback or death. Small or newly planted evergreens are especially vulnerable.
Bagworms will also feed readily on a secondary list of deciduous trees and shrubs, though these plants are generally more resilient. Common deciduous hosts include Maple, Locust (both Honey and Black), Sycamore, Willow, Oak, and Rose. While defoliation can be significant, healthy deciduous trees can generate a new flush of leaves, allowing them to survive an attack with only temporary cosmetic damage. Severe and prolonged infestations over multiple seasons can weaken the tree enough to cause decline.
Effective Control and Removal Strategies
Successful management of bagworm populations depends on proper timing, focusing control efforts on vulnerable stages of the pest’s life cycle. The most straightforward approach for small trees and shrubs is mechanical removal during the dormant season. Property owners should handpick the bags from the infested plants anytime between late fall and early spring, before the eggs hatch in late May or early June.
This manual removal eliminates the overwintering eggs and prevents the next generation of larvae from emerging. It is necessary to destroy the bags by crushing them or sealing them in a bag for disposal, as simply dropping them on the ground allows the eggs to hatch and the larvae to re-infest nearby vegetation. When removing a bag, clip the silk band that attaches it to the branch to prevent future girdling of the limb.
For larger trees or widespread infestations, treatment with insecticides is most effective when the larvae are small and actively feeding, generally from late May through June. The biological insecticide Bacillus thuringiensis (Bt) is a good option for young larvae, as it is a stomach poison that must be ingested and is less harmful to beneficial insects. Chemical insecticides can be used for later-stage, larger larvae, but application is less effective once the bags are fully formed and the caterpillars stop feeding in late summer.