I know that people use cinnamon on their dahlia bulbs over winter to reduce mold on them, but have never heard of using it to repel insects. I think Lee-Roy is correct, if you can identify the problem for sure, then you can fine a remedy. I'm not familiar with the bugs in question. The only things that seem to bother iris rhizomes here are iris borers. I don't think that they are in Calif. though. Maybe some of the California people will have some ideas for you. Good Luck!
