Q: Any idea what could be eating these? Only one caterpillar was found on one leaf.
A: I think the culprit is the larger canna leaf roller. Its adult form is a small skipper moth, which lays eggs on the canna leaf. When an egg hatches, the caterpillar uses silk to pull the edge of a leaf over itself, thus making a leaf roll in which to hide.
As the creatures grow larger, they feed heavily on canna leaf tissue. Many times the “roll” will come apart and the caterpillar will start again, which leads to lots of leaf damage.
The easiest control is to visit your flowers every few days and use your fingers to unroll any leaf flaps you find. Flick the caterpillars off the leaf and make a nearby bird happy to have a tasty snack.