One year something was eating up all my cannas also. They were eaten from the outside of the leaf inward. It was mid summer and I simply cut every one of them down to the ground and threw the plants into a pile of leaves I was burning. They came right back and in greater numbers. I did not have any more problems. I live in zone 9 and we have somewhat tropical weather down here. It's pretty hard to kill a canna here, but I don't know about your area.

It's difficult to guess what might be feeding on your cannas because you do not indicate where you are located. Assuming you are in zone 9, we need to know if that is on the west coast or the deep South. Not sighting the cause during the day indicates the problem likely is a night feeder. This often is slugs or snails depending on your location. It takes a rather large population to do the damage seen. You need to check abound the base of the plants and under any mulch or in crevasses for your problem. Most night feeders like to hide in a dark, moist place in the day. You might try investigating at night with a flashlight. Good luck! Kent

If you Google "Canna Leaf Rollers" you can find out about this pest. BT is a control specifically for caterpillars but you have to use it often to keep them off. I have the same problem. There is also a disease that affects Cannas, I love them but it's a constant battle to keep them healthy. I have used Sevin dust for them and it works but also have to reapply often to get good results.

looks like what i call a cut worm ... link to google pix below ... they attack my hosta while there are still furled up .... and then they unfurl .. showing damage that seems to mirror each side of the leaf .... like on your second picture .... scratch around the soil/mulch .. and see if you can find a matte brown bug of about one inch long .... they hide during the day ... i am in z5.. and no bug expert.. just throwing out an idea... ken Here is a link that might be useful: link