Without question, systemic insecticides can provide the best control for leaf roller control in heavy infestation areas of the country. Application time and intervals will vary relative to the form of the product. Granular products applied to the soil require a longer time taking effect and can be somewhat less effective over the growing cycle. It may be necessary to reapply them during the growing season. The liquid systemic products become effective much quicker but require a bit more care concerning drift and contact of non-targeted beneficial insects. I would recommend very early or very late daytime applications. In areas that have heavy infestation yearly, start applications of the granular at planting time and reapply according to instructions (may be every 4 to 6 weeks?). Liquid applications can begin after the first leaves appear or at the first sign of damage. Reapplication typically can vary from as often as every ten days to three weeks. Re-infestation typically becomes greatest by mid to late summer, requiring the more frequent applications.
Please exercise care and follow label instructions. Kent

Thought I'd pass on this little tidbit I've used and it worked for me. Also, it's organic. About 3 years ago I too had leafrollers on my Cannas. Read in the organic forums here at Garden Web that sprinkling flour on them would get rid of the little creatures. Thinking that it would'nt work. Tried it anyway. Guess what. It Worked! That was 4 years ago and no more since. Be sure to put it under them too. Just a light sprinkling.

Interesting use for flour??

It would be helpful to know when you applied your flour (what kind) and how often. Was this application only once or throughout the season? Readers do need to know what area of the country you reside; there are large areas of the U S where the leaf roller is not a serious problem if they exist at all.

I question the value of simply sprinkling flour on the leaves of cannas considering that the lesser leaf roller larva lives inside the tightly curled whorl of a canna leaf; this is why even contact insecticides applied to the leaves are of little use.

This organic control sounds great if we had more information and can duplicate your suggestion in the most prone areas of yearly infestations.

Kent

I don't find it too difficult or time-consuming to unroll the leaves and kill the caterpillars, but my couple patches of these only consist of about 25 tops each. Since I kill them when I find them (and check often) they are sparse.

If I did find a ton of rolled leaves one day, I would try using a garden syringe or turkey baster or little funnel to put a few drops of rubbing alcohol down into the webbed leaf - you can definitely see which leaves have a caterpillar in them. I think that would make it come out if it didn't just kill it. Next time I find one, I will try it. The leaf would still need to be rescued though, the web slit along the leaf edge, so the leaf can unroll. Often they look good enough to leave attached if you notice the web soon enough.