I have numerous beds of bearded iris, some of which have been attacked this year by iris borer.
I have started lifting and cleaning them.
My question is: How do I treat the soil so that I can replant them in the same beds?
I don't live in a borer prone area, so I am not the best person to answer this. What I have heard is that Bt used on the plants early on can help control them. Also there are predetory nemetodes that can be used when the larvae are out of the rhizome and going into the soil for the next step in their development I think. Some people have used imidicloprid (a systemic insecticide) in the spring. I am not sure about what kind of soil treatment there is other than keeping the dead foliage cleared away in the fall and winter.
Here is a link that might be useful: Page on borer control