These little cactus bugs are called Cactus Coreid, or Chelinidea vittiger. I observed them while out on the trail at Inks Lake State Park, about an hour outside of Austin.
As their name implies, they are associated with cacti, specifically Opuntia, which includes prickly pear. Eggs are laid on the underside of Opuntia species, making them very host specific. They feed on the vascular fluid of the host plant they were born on. They have been observed to starve before finding a new host plant if their original was destroyed. Due to this host specificity, their range extends wherever Opuntia species are found.
My iNaturalist post can be found here.
Source: http://www.inaturalist.org/taxa/216651-Chelinidea-vittiger