I use Sevin spray on my cannas. I spray them about once every ten days. There are stronger chemicals you can use on them, but the sevin works fine. I would not cut them back so severely. The moths are laying eggs on the plant because they love the plant. They will not go away unless you make the plant less attractive. Pick the worms off, cut the damaged leaves off, and start spraying to control the larve from the moths. I started using smell assault this year with a lot of areas. I have rosemary planted in the pot with my cannas to see if that will deter worms (leaf rollers). My canna issue thus far is canna rust (fungus) from the rain we've been having. Cannas are one of those needy plants I forget are needy until I buy them, and then whine all summer that I bought them.
Thanks, but I've already mowed them down. They always seem to come back no matter how much I neglect them. (Plus, I'm a reluctant gardener so you won't catch me picking worms off of anything...haha. ) So your cannas are in a pot? How does that work out? I have a relatively large bed of them, all along our back fence.
They don't seem to mind being in a pot. Since my garden is in a constant state of construction, I put a lot of stuff in pots to camouflage the blocks and bags of mulch behind it. I have pampas grass in a pot, tiger lilies, purple sage, coneflowers, and phlox. We have thick clay so digging out beds is not an easy task. I buy the plants as I find them, and propagate them to get more so as I expand, I have plant material to put down. I don't know why anyone would be put off picking worms off a plant...