Well, from what I've read in having to deal with my select little groupings of iris... the iris borer moth lays its eggs in the foliage in late summer, early fall. Clean up, cut down the old foliage in the fall and you don't have any place for the moth eggs to winter over, become the actual borer caterpillar, travel down the foliage and eat away at the rhizome.

Dig them up, hose them off or soak in a pail if you find active borers, cut away any rot and replant. Same place is fine... they're not in the soil. But clean up in the fall is key.

yes, i just fought my own battle with the borers, luckily of the 80-100 individual iris rhizomes i have (i know, soooo many), i dug up 6 or so that looked suspicious, i found one dead borer in the rhizome, pulled one out before it tunneled into the rhizome and pulled out 2 live ones from the rhizome promptly squashing them angrily, one was really fat and probably close to pupating. so far all the ones i replanted seem healthy, hopefully they stay that way until next year. I have doubles of basically everything i have (except the new ones obviously) so i'm not worried about deaths, i'd just prefer to see them bloom instead of die.

ick.

You can dip the plants in a 10% bleach solution for a few minutes, too.

The iris borer has almost made me give up on bearded iris...almost. I do all "the right things" to keep them at bay, but they always seem to find their way back.

Funny, I was just out cleaning up the area, and noticed how much better they look this year. I don't like to use chemicals, but last fall and again in the spring, I sprinkled a little GrubEx on the area. Not a perfect result, but certainly a very good result.

My sympathies...iris borers are not one of the happiest aspects of gardening.