Salvage the rhizomes by digging out the pinkish colored iris borer (Macronoctua onusta) larvae with a pocket knife and trimming away damaged tissue. Discard heavily infested rhizomes.

Because their feeding creates opportunities for infection by bacterial soft rot (Erwinia carotovora), stave off disease outbreaks by soaking the rhizomes for about a half-hour in a 10-percent household bleach solution. Afterward, dust the trimmed plants with powdered sulfur and lay them in a shady place for several hours to allow the cut ends to dry and heal. Before you replant the irises in their original location, comb the soil carefully for signs of brown pupal cases. Collect and destroy them to prevent the dusky brown adult moths from emerging in the fall and laying eggs on or near your plants.

In late fall or early winter, clean up and remove old, dried iris leaves, stems, and other debris to eliminate overwintering eggs. Continue to combat this pest when the eggs hatch in mid- to late spring. The tiny larvae crawl up the iris leaves and feed, leaving telltale notches. Then they enter the leaves, producing pinpoint holes. As they slowly mine their way down toward the rhizomes, leaving ragged, water-soaked leaf fans in their wake, squash them by pressing the leaves between your thumb and forefinger.

Sanitation is the first line of defense. If your iris bed is heavily infested and you follow the above plan, consider reestablishing the irises in another area. With heavy infestation, sift the top soil of the iris bed through a quarter-inch square sieve to remove and destroy as many pupa as possible.

Contact your local Cooperative Extension Service for chemical control recommendations.