Answer: You're dealing with borers, the larvae of large flylike insects. There's not much you can do to save a rhizome or corm infested with these pests, but you can try to avoid another attack by practicing good garden sanitation each year. After the leaves of your iris and glads die down, rake and remove them from the garden, to expose the eggs to birds and weather. This should keep the insect eggs from hatching and the resulting larva from feasting on the below ground plant parts. I'd dig and toss the infested rhizomes and corms, and plant new iris and glads in a different garden spot to prevent reinfestation.
Best wishes with your garden!
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