The bagworm is a type of moth larva that presents a significant threat to landscape plants due to its feeding habits. This pest, scientifically known as Thyridopteryx ephemeraeformis, spends its larval stage encased within a protective structure made of silk and plant material. The caterpillar carries this spindle-shaped case, which can grow to two inches long, as it feeds constantly throughout the growing season. Understanding the bagworm’s diet is the first step in protecting various trees and shrubs from the damage this insect can inflict.
Preferred Diet: Conifers and Evergreens
The bagworm exhibits a clear preference for coniferous and evergreen plants, which represent the primary and most damaging portion of its diet. These pests are particularly attracted to the needles and scale-like foliage of popular landscape specimens. Common host plants include juniper, arborvitae, cedar, pine, spruce, and Leyland cypress, where the larvae feed on the green photosynthetic material.
Feeding on evergreens is especially destructive because these plants do not readily regenerate lost foliage from old wood. Once a branch or section of the canopy is completely stripped of its needles, that portion of the plant dies back permanently. A heavy infestation that causes widespread defoliation can kill a smaller evergreen tree or shrub within a single season.
The caterpillars often begin feeding at the top of the host plant, causing the canopy to thin and brown from the top down. This feeding habit means that by the time the damage becomes noticeable at eye level, a significant portion of the plant may already be irreparably harmed. The larvae incorporate fragments of the consumed foliage into their silk bags, which aids in camouflage.
Opportunistic Feeding on Deciduous Plants
While bagworms prefer evergreens, they are highly adaptable herbivores capable of feeding on over 125 different plant species, including deciduous hosts. These broadleaf plants become a food source, particularly when primary coniferous hosts are scarce or during heavy infestation periods. The bagworm’s ravenous appetite is not confined to one type of plant, allowing them to thrive in varied environments.
Examples of deciduous trees and shrubs that bagworms feed on include maple, oak, sycamore, rose, elm, and black locust. The damage to these broadleaf plants is often less severe than to evergreens because deciduous species can typically produce new foliage after being defoliated. This ability to refoliate makes them more resilient to a single season of feeding pressure.
Repeated severe defoliation of deciduous plants over multiple years can significantly weaken the tree or shrub. This weakening makes the plant more susceptible to other stressors, such as drought, disease, or attack from borers. Even on these more resilient hosts, a persistent bagworm population can compromise the overall health and vitality of the plant.
Recognizing Damage Caused by Feeding
Identifying bagworm damage involves observing specific visual cues on the plant’s structure. One of the earliest signs of injury on evergreens is the browning or stressed appearance of needles at the tips of branches. Tiny, newly hatched caterpillars etch the surface of the needles as they feed, causing this initial discoloration.
As the larvae grow, they consume more substantial amounts of foliage, leading to thinning of the plant canopy. On evergreens, this widespread feeding can result in a symptom called “bronzing,” where green sections turn a permanent bronze or brown color. The most obvious sign is the presence of the tough, spindle-shaped bags hanging from the twigs and branches, which are constructed from silk and pieces of the host plant.
The caterpillars feed by extending their head and thorax out of the top opening of their bag, creating defoliation that can leave behind only the midribs of leaves on broadleaf plants. If left unchecked, this feeding can lead to dramatic changes, making the plant look sickly and pale, often starting in the summer months.