Harmostes reflexulus (Say) is a bivoltine inflorescence specialist that tracks a succession of host composites (Asteraceae), typically remaining on a host until seed set. During fieldwork from 1998 to 2011, the rhopalid′s host plants were recorded (124 collections) in Arizona, Colorado, Nebraska, New Mexico, Oklahoma, Oregon, Texas, and Wyoming. Twenty-six species of the subfamily Asteroideae on which nymphs, mating pairs, or both were found are considered host plants; 25 are new host records. Adults were taken on 12 additional species of asteroid Asteraceae. Host composites were white- and yellow-rayed herbs and subshrubs (Ericameria nauseosa is a shrub) that are mainly perennial. The Astereae (11 spp.) and Helenieae (7 spp.) were best represented among asteroid tribes that the rhopalid used as hosts; other host tribes were Anthemideae (1 species), Bahieae (2 spp.), Heliantheae (1 species), Madieae (1 species), Senecioniae (2 spp.), and Tageteae (1 species). In the Astereae, hosts mainly were in the subtribe Machaerantherinae (5 spp.); in the Helenieae, all seven hosts were in the subtribe Tetraneuriinae. Composite tribes such as Cardueae, Cichorieae, Eupatorieae, and Gnaphalieae were not among the rhopalid′s observed hosts. Records of H. reflexulus from non-asteraceous families are considered incidental or to reflect only a use for shelter or adult feeding.