Hi S. Bird,
Iris borer can be controlled. For a natural rememdy:
"Beneficial nematodes are a natural option to help control some boring insects. These nematodes are microscopic worms that seek out and kill the caterpillars by introducing a bacteria. Research results using nematodes have been inconsistent, although they can potentially be effective in reducing iris borer populations.
Two species of nematodes are commercially available from natural garden supply catalogs or your local garden center. Both species (Heterorhabditis and Steinernema) have been shown to attack iris borer. Nematodes need a moist environment to survive and move. The easiest time to apply is during a rain shower. Lacking rain, water iris before and after applying nematodes. Apply nematodes when iris leaves are almost fully expanded but before flowering. Repeat the treatment approximately one week later. Nematodes are perishable; check the label for the expiration date."
Also see this site.
http://www.ppdl.org/dd/id/iris_borer-iris.html
Newt