This globular, yellow creature is the larva of a swamp milkweed leaf beetle, Labidomera clivicollis. Leaf beetles, in the family Chrysomelidae, tend to be named after the plants on which they specialize; as you may guess, the swamp milkweed leaf beetle eats the leaves of the swamp milkweed plant, Asclepias incarnata.
These larvae will grow up into reasonably attractive, red and black beetles, which also eat swamp milkweed, and, because of this, are full of milkweed toxin and, at the very least, taste terrible. The red and black coloring, called aposematic coloration, warns potential predators away. This might work for the adult beetles, but the relatively plain (and also relatively-low-in-milkweed-toxins) larvae are food for damsel bugs, stink bugs, and hoverfly larvae. As eggs, they are also (potentially) food for each other, and for mom — female swamp milkweed leaf beetles and larvae “dabble in egg cannibalism” (the males do not).
I’d write more about this, but the BugLady really has covered it, and is much more entertaining than I am. 🙂