This is the caterpillar of a Pipevine Swallowtail (Battus philenor). Their food plant is pipevine and other milkweeds which
confer toxicity to the caterpillar. The bright coloration serves as due warning to would-be predators. And this one
was out in the open on a stalk of fountain grass. It could be ready to pupate because I didn't see any pipevine nearby.