August was SO HOT here in SoCal. I know that Sept and sometimes Oct are hot too. But I am happy to have survived another August and to have it behind us.
Every so often I find Whitefly on a plant. They do like Canna Lily leaves. Whitefly is a sucking insect found on the underside of leaves. See the gross pics below. They fly around in clouds if disturbed. The will damage the plants. I got grossed out just starting to read the wikipedia page. wikipedia Whitefly Ew! Usually I don't spray insecticide on Cannas because you're supposed to cut off the spent flowering canes all the way to the ground after blooming anyway. So I just cut them off and deposit them in the trash. I once had to cut down a whole 20 foot long row of them that had whitefly. I had to wear a bandana over my mouth because of all the flying nonsense. And they make all this fluffy cottony mess too.
This pic(stolen from the internet) is a close-up view of the little pests. They aren't really that gross by themselves.
If you see them on your plants, you could spray with an insecticide found at your local nursery. Make sure by reading the label that it is safe to spray on the plant you have.