Ask Mr. Smarty Plants

The  Aristilochiaceae, or Birthwort family is a family of flowering plants with 8 genera and about 400 species. The type genus is Aristolochia, also known as pipevine. Many members of Aristolochia contain the toxin aristolochic acid which discourages herbivores. Pipevine swallowtail butterflies lay their eggs on pipevine, and the caterpillars dine on the plant but are not affected by the toxin. This causes the adult butterfly to taste bad to predators, thus affording them some protection. The adults are pretty consistent in their choice of plants where they lay their eggs.

Our Native Plant Database   lists 8 species of Aristolochia that are found in the US, six of which occur in Texas. Four of these occur in south Texas, and two species are known to occur in Hidalgo County.

Aristolochia erecta (swanflower), larval host for Battus philenor, Hidalgo County.  This is very likely the species attracting Pipevine Swallowtails to your garden.  The plants often grow in grassy areas and because they're very small with long narrow leaves, they are almost invisible in grass.  Watching a female Pipevine Swallowtail fluttering about a grassy area and then inspecting the places she alights will often reveal the location of Swanflower in the grass.

Aristolochia pentandra (Marsh's dutchman's pipe),  not listed as a larval host, Hidalgo County.  This species' habitat is restricted to wetland areas.

Aristolochia tomentosa (woolly dutchman's pipe) , larval host for Battus philenor,  Bexar County.

Aristolochia serpentaria (Virginia snakeroot) , larval host for Battus philenor Edwards and Bandera Counties.

The other Pipevine swallowtail that occurs in Hidalgo county is Battus polydamas, but its larval host is listed only as Aristolochia species.  This butterfly prefers the more toxic pipevines as larval host plants, so it is likely that Polydamas Swallowtails are mainly attracted to Marsh's Dutchman's-pipe and exotic Aristolochia species grown in private gardens.