Snail-Case Bagworm Moths: these caterpillars use silk, sand, and other bits of debris to build spiral-shaped cases around themselves, and this particular subspecies is entirely female
The scientific name of this species is Apterona helix, but it's commonly known as the snail-case bagworm moth, because the caterpillars create spiral-shaped cases that resemble snail shells. The cases shield the developing insects from predators and parasites, and they serve the same purpose during the pupal and adult stages of development.
Above: snail-case bagworms in their cases
Each case is constructed from sand, soil, and bits of debris that are pasted together with silk; the structure initially has a straight and relatively narrow appearance, but more material is added as the caterpillar grows, gradually causing the case to become coiled.
Above: more of the spiral-shaped structures
The North American variant of this species is entirely female, and it reproduces asexually by engaging in a process known as parthenogenesis. The same all-female subspecies can also be found throughout most of Central and Western Europe.
The adult moths are completely wingless, and they have bald, grub-like bodies with only vestigial legs. Once they reach maturity, they lay their eggs inside their spiral-shaped cases and then finally emerge from the structures, dying shortly thereafter.
As this article explains:
This insect reproduces parthenogenetically, is wingless in all stages, and spends its entire existence within its case, which it constructs using silk, soil particles, and fecal matter. The bagworm case expands in a helical or coil-spring fashion as the larva grows. The completed bagworm case typically has three whorls or coils and is approximately 4mm in diameter.
When it reaches maturity, the larva, carrying its case, will climb any available tree, shrub, fence, building, or vehicle, attach itself to the surface with silk, and pupate within the case. The adult later emerges, lays eggs in the case, and then crawls out to die.
Above: a tiny snail-case bagworm resting on a strawberry
The snail-case bagworm moth is native to Europe and Western Asia, but it was accidentally introduced to the United States in the 1940s. It's now recognized as an established species in many states throughout the country, and it has also been established in several Canadian provinces.
This species is sometimes referred to as Apterona helicoidella or Apterona crenulella. Some sources use these names to identify different populations (i.e. Apterona helix in North America, Apterona helicoidella in Central Europe, and Apterona crenulella in Southern Europe) but they are officially regarded as members of the same species.
Penn State University: Snailcase Bagworms
iNaturalist: Genus Apterona
South Dakota State University: The Snailcase Bagworm
The Natural History Society of Upper Lusatia: Spread and Lifestyle of the Snail-Shell Bagworm
Washington State University: Apterona helix (PDF)
British Columbia Wine Grape Council: Snailcase Bagworms
Agro News: Those Small, Spiral-Shaped Mud Cases Could Contain Snailcase Bagworms
Colorado State University: The Snail-Case Bagworm (PDF)
Proceedings of the Entomological Society of Washington: Apterona helix: a Palearctic Bagworm Moth in North America
