I try to watch early in the season (April/May/June) for those streaks coming down the center of the leaves, in the creases where the leaves intersect. That's a sign that the iris borers have started their trek down the leaves into the rhizomes. If I see those streaks I get out the insecticide and spray the leaves to saturation, hoping that it will run down into the leaves and kill them. I use Sevin, Malathion, and Merit. The Merit is in Bayer 3-in-one, a systemic that I use often on the Hibiscus, so stop and spray the Iris once in a while too.
If I get a really bad infestation I also have a small amount of Lindane that I will spray - that really does the job, actually I've heard it lasts a few years, so I use it very sparingly.
Now, if you do miss the early signs, you will often see the tunnels get bigger, with caterpillar poop coming out of the leaves as they grow bigger and bigger. At that point you can cut down the foliage to try and find it and then squish it. By now, this time of year, the borer will have reached the rhizomes. Best way to get them now is to lift, kill it by hand, then replant, although if you really have a lot of them you might try saturating the soil with an insecticide. I agree, you really don't need to throw away all of it, as there are usually some lateral new rhizomes that will still be okay. The really bad ones can be pitched, though, as they often get a secondary fungus infection when really badly damaged.
In the fall it is highly recommended to cut down and throw away ALL of the old foliage from the Iris. That is where the borer eggs will overwinter, so throwing away the foliage can cut back on the population for next season. I do notice that areas that were bad one year tend to get worse the next if I don't get after them.
Hope that helps a little too.