This action introduces a bacteria that rots the rhizomes. Start control of these pests in the fall by removing all dead iris foliage where eggs may have been deposited. It’s probably best to throw them in the garbage instead of composting them. If you start to see signs of feeding on the leaves you can actually squish them, they should be at the bottom of the lesion feeding down towards the rhizome. If there are a lot of lesions remove and dispose of the leaf in the garbage, not the compost pile. You can also spray acephate sold as Bug B Gon: Japanese Beetle Killer when the leaves are about four inches tall.
If you have a problem with the larva in the rhizomes you will have to dig them up. You can actually kill the larva by poking them with a piece of wire. You will have to cut away the portions that have begun to rot from the bacteria then dip the undamaged portions in a solution of 1 part of household chlorine bleach and 10 parts of water before you replant them.