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

I use BT (Bacillus thuringiensis) on my grape vines to keep the grapeleaf skeletonizers off. It works well but you have to apply it every few weeks and after rain. Supposedly it works with all caterpillars or worms that eat plants.

Interesting predicament. I don't have any cannas (or arrowroot). From UofF: http://entnemdept.ufl.edu/creatures/orn/brazilian_skipper.htm Let us know if any of the below in not consistent with your experience in the coming weeks. ''Mechanical control of leafrollers on a few canna plants in the landscape is relatively easy. Simply examine your cannas every couple of days, and squish any small shelters that appear. This will reduce populations of the leafroller before they can cause large amounts of defoliation. Chemical control is difficult because larvae are protected from direct chemical exposure within their leaf rolls. If deemed necessary, chemical control should be attempted when larvae are small and are still actively feeding on the leaves. Bacillus thuringiensis (Bt) products are probably the best choice for chemical control. Follow the application instructions on the label of the specific Bt product. Stickers will probably need to be added to the spray because the thick waxy coating on canna leaves repels water.''

That’s interesting .. I already have some spinosad but didn’t realise, it is marketed here as a fruit fly control, see the blurb below. Nature’s Way Fruit Fly Control has been included in this range because it’s based on spinosad, the naturally-derived insecticide found in Yates Success. Yates Nature’s Way Fruit Fly Control works by tricking the insect into absorbing the spinosad that’s been combined with an irresistible bait. After she emerges from her pupal case in the soil, the female fly looks to feed on a source of protein before she lays her eggs. When she eats the protein and sugar bait in Nature’s Way Fruit Fly Control, she also ingests a dose of the insecticide, which means she never makes it to the egg-laying stage. Fruit fly baits are nothing new but many have used a hormone that only attracts male flies (they’re the ones that don’t lay the eggs!) or have been based on a nasty chemical. Now Yates Nature’s Way Fruit Fly Control has a low toxic formulation that really works. also found this because spinosad is highly effective against diamondback moth, but because it also controls several other important lepidopteran pests such as heliothis (Helicoverpa spp.), cabbage white butterfly (Pieris rapae), cabbage centre grub (Hellula hydralis) and cabbage cluster caterpillar (Crocidolomia pavonana) at rates which provide growers with excellent value for money. Spinosad is highly active against loopers (Chrysodeixis spp.) and affords some control of cluster caterpillar (Spodoptera litura) and onion thrips (Thrips tabaci). So so if my rose slugs ever start on my buds I’ll use this. Might even do it on the climbing nahema as I can only squish so high! And they do shred the leaves to pieces ...