Zeller, 1839[Synonyms: argrammella]
Wingspan 8-10 mm.
Until 1955, and sometimes later, the name of this species was incorrectly given to the ubiquitous rush feeder, C. alticolella. It is widespread and locally common in England, but less so in Wales and Scotland.
It flies mid May to June at dusk and night. The adults resemble several other Coleophora species, and are most reliably identified by dissection of the genitalia.
At the larval stage it can be distinguished from other Juncus feeders by its grey case heavily encrusted with blackish frass and debris, with paler longitudinal expansion grooves.
C. caespititiella is seen less frequently than other rush feeders, partly because it is more secretive. In August it lives, without a case, concealed in a rush seed. In September it feeds on Juncus seeds from its case, 4 to 5 mm long, often hidden in the seedhead. In late September or October the full fed larva either leaves the foodplant or hides itself in the seedhead for the winter.
Foodplants include Juncus subuliflorus/conglomeratus, J. effusus, J. articulatus, J. inflexus and J. gerardii.