The plaster bagworm (Phereoeca uterella) is a common household pest often found in warm, humid environments. This insect is the larval stage of a small moth, easily recognizable by the protective, portable case it carries. While homeowners often worry about potential health dangers, plaster bagworms do not bite humans or pets. They are non-toxic, non-aggressive, and are not known to transmit diseases or cause direct physical harm.
Identifying the Plaster Bagworm
The most distinctive feature of this pest is its protective larval case, a flattened, spindle-shaped structure. This silken bag is typically gray or brownish and is camouflaged with surrounding debris, such as lint, sand, insect droppings, or fragments of plaster and stucco. When fully grown, the case usually measures between 8 and 14 millimeters long, roughly the size of a small pumpkin seed.
The larva is rarely seen, as it remains inside this portable shelter, extending only its head and legs to move and feed. The case has openings at both ends, allowing the larva to turn around within the structure. The small, pale larva has a brown head and carries its case as it forages slowly across surfaces like walls, ceilings, and floors. Homeowners can confirm an infestation by spotting these unusual, slow-moving, seed-like objects.
Addressing the Biting Concern
The plaster bagworm poses no direct physical threat because its mouthparts are not designed for piercing skin. As a larva, its primary function is to chew and consume various organic and fibrous materials within the home. They lack the necessary apparatus for biting defensively or drawing blood from a host.
These insects are scavengers and detritus-feeders, meaning their diet is composed solely of non-living organic matter. Any sensation mistaken for a bagworm “bite” is likely due to another insect species or misidentification. The larvae are largely sedentary, only moving to find food, and are not aggressive toward people or animals. The primary concern with their presence is the damage they can inflict on household goods.
Property Damage Caused by Bagworms
Since they do not bite, the true impact of an infestation lies in their feeding habits as they consume materials within the home. The larvae eat items containing keratin, a protein found in natural fibers like wool, silk, and felt. This feeding results in cosmetic damage, such as small, irregular holes in stored clothing, carpets, or other textile items.
Their diet also includes detritus like hair, lint, dead insects, and spider webs, which they seek out in undisturbed areas. They incorporate this debris into their silken cases for camouflage and reinforcement. In humid conditions, they may also feed on fungi or mold growth. Their movement can sometimes leave trails of grazing marks on painted walls or wallpaper, but they do not cause structural damage to the wood framework of a home. Their presence indicates a need for cleaning and moisture control.
Effective Removal and Prevention
Eradicating an infestation begins with non-chemical methods focused on physical removal and environmental control. The most effective first step is using a vacuum cleaner to suction up all visible cases from walls, ceilings, carpets, and corners. Thorough vacuuming should also target areas where the larvae’s food sources accumulate, such as under furniture and along baseboards, removing lint, dust, and spider webs.
Controlling the environment is equally important, as plaster bagworms thrive in warm, high-humidity conditions. Using dehumidifiers or air conditioners to lower the moisture content in damp areas like basements and closets makes the environment less hospitable. Reducing outdoor lighting and sealing cracks can prevent adult moths from entering and laying eggs. In cases of severe, persistent infestations, a targeted insecticide spray labeled for indoor use against case-bearing moths may be necessary, but only after extensive cleaning efforts.