According to microleps.org, "Caloptilia murtfeldtella ... larva[e] feed[ ] on Penstemon spp. ... [T]he larva ... induces and lives inside a stem gall; one or more galls may occur per stem, but only one larva inhabits each gall. The galls occur low on the stem, usually from ground level to a few inches above ..." For more information and photos, see the microleps webpage linked above and also the original source: Braun, A. F. 1922. Microlepidoptera: notes and new species. The Canadian Entomologist 54: 90-94.