Size equivalence, seasonal synchronicity, geospatial sympatry, habitat specificity, and host-searching behavior implicate the spider wasp Chalcochares hirsutifemur (Banks) (Hymenoptera: Pompilidae: Pompilinae) as an obligate parasitoid on species of the wafer-lid spider genus Aptostichus Simon (Araneae: Mygalomorphae: Euctenizidae) on coastal sandy back dunes in San Luis Obispo and Santa Barbara counties, CA. This is substantiated by 2010–2022 macrophotographs, videos, and field observations. Such host evidence supports recent unpublished phylogenomic studies that place Chalcochares as a sister genus of the spider wasp tribe Aporini, in which all species are known obligate parasitoids on trapdoor spiders and related Mygalomorphae. Chalcochares hirsutifemur and C. engleharti (Banks) are separated based on morphological, geographic, and probable host spider differences. Resource partitioning on the coastal sand dunes between C. hirsutifemur and three species of smaller Aporus Spinola is proposed.